Test engineer

A test engineer is a professional who determines how to create a process that would best test a particular product in manufacturing and related disciplines, in order to assure that the product meets applicable specifications. Test engineers are also responsible for determining the best way a test can be performed in order to achieve adequate test coverage. Often test engineers also serve as a liaison between manufacturing, design engineering, sales engineering and marketing communities as well. 
Test engineers can have different expertise, which depends on what test process they are more familiar with (although many test engineers have full familiarity from the PCB level processes like ICT, JTAG, and AXI) to PCBA and system level processes like board functional test (BFT or FT), burn-in test, system level test (ST). Some of the processes used in manufacturing where a test engineer is needed are:
Early project involvement from design phase.
Ideally, a test engineer's involvement with a product begins with the very early stages of the engineering design process, i.e. the requirements engineering stage and the design engineering stage. Depending on the culture of the firm, these early stages could involve a Product Requirements Document (PRD) and Marketing Requirements Document (MRD)—some of the earliest work done during a new product introduction (NPI). 
By working with or as part of the NPI group, a test engineer ensures that a product is designed for both testability and manufacturability. In other words, to make sure that the product can be readily tested and built.
The following are some general rules to ensure testability and manufacturability of a product:
By following the general rules above, test engineers minimize future surprises (like adding extra components, re-layout of the boards, etc.) which drives up costs and development delays of the final product.
Working with cross platform teams, hardware and software team.
Often people take shortcuts to be able to deliver final products. Because of these shortcuts, the product's manufacturability and testability becomes complicated (inability to read and write information, creating deviation from the process, etc.) which impacts the manufacturing complexity of a product. Because of this complexity, bottlenecks in the manufacturing and delivery schedule delays are introduced.
With this in mind, test engineers always get involved in the following reviews as well:
Yield maintenance.
Products' yield plays a very important part during their lifespan. There are usually three stages for a product, engineering, initial production (IP) and full production (FP). 
In addition, yields will show if another process needs to be introduced (e.g., because processes already used cannot capture certain test errors). Yields can also decide if an existing test process can be trimmed down (step-wise or time-wise) or even fully eliminated. E.g., if the ESS errors can be captured during the 3rd hour, test time can be cut down from a normal 24 hours down to maybe 4. Or if a process consistently yields 100% during a 15-month period, teams can get together and decide to eliminate that process at all.
Test automation.
Test automation refers to the automation of the process to test a product through the use of machines. Depending on the product, the machines that we are referring to could mean a combination of Automatic Test Equipment (ATE), handler, interface board, and test program that drives the ATE, as with the case of the IC chip testing.
Test automation is a big part of a test engineer's job. 
The whole intention of automating the test is as follows:
Overall, this drives manufacturing reliability and quality at the end of the line making sure that all units shipped out to customers are well tested, stressed, filtered out of any errors, and configured properly.
Defining standard test documents.
Following are some of the documents that the test engineers maintain or define:
Contract manufacturer.
A contract manufacturer (CM) also provides a test engineer for their customers. The function of these test engineers varies depending on the level of support they provide for their customers: providing "interactive and first level of defense"-only support or providing partial or ground-up solutions.
Providing interactive and first level-of-defense support.
Providing "interactive and first level-of-defense"-only support is the usual job of the CM TE. Here are some typical job functions for a CM test engineer:
Because of their close involvement with the test line, they monitor the products going through the line and inspect the failed boards to decide if it really failed or if the failure was just caused by some improper test setup. Some examples of these false failures are:
Providing partial or ground up solutions.
There is a small number of companies who prefer to outsource their test engineering work to their corresponding CM. In that case, the CM TEs will be in charge of providing the test automation solution, test fixture design, yield gathering plus the usual interactive and first level of defense for their customers.
Of course, outsourcing test solutions to the CM has its pros and cons. 
Some of the advantages are:
Some of the disadvantages are:
Because it is hard to find a test engineer who knows every aspect of testing methodology (from PCB tests like ICT, JTAG test, flying probe test, and X-Ray test to PCBA test which includes writing test automation from functional test to FQA test among others), companies usually outsource part of the development of this missing test piece to their CM. For example, if none of the in-house TEs know much about ICT fixtures, they will ask their CM to develop the ICT test solutions for them instead.