user6024: I'm a bit confused on how testing stuff for Bitcoin on the testnet currently works.
For example, bitcoin core 0.10 enables only 40 bytes in OP_RETURN, but I'm not sure if 0.10 is the latest version allowed on the testnet.
I know that there are OP_RETURN transactions in the testnet block chain that are up to 64 bytes, as seen in this transaction:
{
  "transaction_hash": "91615716a4d3c42cafc240b594d79782709c2c359bd0d5a1d30fa2ee8294bcd3",
  "hex": "37356365626362356462366363636537383565633837646134353564373332646162653661313462636362623235393338643465663163373539346232303934",
  "text": "75cebcb5db6ccce785ec87da455d732dabe6a14bccbb25938d4ef1c7594b2094",
  "receiver_addresses": [

  ],
  "sender_addresses": [
    "mmHWy49WWGFQgmeZt7wYKvWSh7Szi9XQAm"
  ]
}

source
Is the code for the testnet differences within the most recent bitcoin github repo, or is it a separate repo I must download, compile, and point towards the testnet?
Some clarification on how this works would be great.

user2306: 
For example, bitcoin core 0.10 enables only 40 bytes in OP_RETURN, but I'm not sure if 0.10 is the latest version allowed on the testnet.

40 bytes in OP_RETURN has always been legal. It was just non-standard, meaning that the standard client wouldn't mine or relay those transactions, but would allow them if they were already in a block.
However, testnet disables all standardness rules.

Is the code for the testnet differences within the most recent bitcoin github repo, or is it a separate repo I must download, compile, and point towards the testnet?

It's the same program with the -testnet switch passed at the command line. (Or in your bitcoin.conf. Either works.)

6024: <p>I'm a bit confused on how testing stuff for Bitcoin on the testnet currently works.</p>

<p>For example, bitcoin core 0.10 enables only 40 bytes in <code>OP_RETURN</code>, but I'm not sure if 0.10 is the latest version allowed on the testnet.</p>

<p>I know that there are <code>OP_RETURN</code> transactions in the testnet block chain that are up to 64 bytes, as seen in this transaction:</p>

<pre><code>{
  "transaction_hash": "91615716a4d3c42cafc240b594d79782709c2c359bd0d5a1d30fa2ee8294bcd3",
  "hex": "37356365626362356462366363636537383565633837646134353564373332646162653661313462636362623235393338643465663163373539346232303934",
  "text": "75cebcb5db6ccce785ec87da455d732dabe6a14bccbb25938d4ef1c7594b2094",
  "receiver_addresses": [

  ],
  "sender_addresses": [
    "mmHWy49WWGFQgmeZt7wYKvWSh7Szi9XQAm"
  ]
}
</code></pre>

<p><a href="http://explorer.chain.com/transactions/91615716a4d3c42cafc240b594d79782709c2c359bd0d5a1d30fa2ee8294bcd3#!transaction-op-return" rel="nofollow">source</a></p>

<p><strong>Is the code for the testnet differences within the most recent bitcoin github repo, or is it a separate repo I must download, compile, and point towards the testnet?</strong></p>

<p>Some clarification on how this works would be great.</p>
 2306: <blockquote>
  <p>For example, bitcoin core 0.10 enables only 40 bytes in OP_RETURN, but I'm not sure if 0.10 is the latest version allowed on the testnet.</p>
</blockquote>

<p>40 bytes in OP_RETURN has always been legal. It was just non-standard, meaning that the standard client wouldn't mine or relay those transactions, but would allow them if they were already in a block.</p>

<p>However, testnet disables all standardness rules.</p>

<blockquote>
  <p>Is the code for the testnet differences within the most recent bitcoin github repo, or is it a separate repo I must download, compile, and point towards the testnet?</p>
</blockquote>

<p>It's the same program with the -testnet switch passed at the command line. (Or in your bitcoin.conf. Either works.)</p>
