Since the flickering was visible during the display self-test, there is a hardware issue with the display or cable. (I hope you ran the dedicated display test as I indicated, holding D as you power up.) It may be an issue with the inverter, which would require a display replacement. If it were my laptop, I would disconnect the battery, discharge flea power, and disconnect and reconnect the display cable, just in case it's not solidly connected. I don't think that's the issue here, but I'd want to be sure. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.) If you do need to replace the display, Parts-People sells genuine Dell parts and they have some displays for your laptop model. If you live in the US, Parts-People (specialists in supplying genuine Dell parts and repairing Dell computers) offers a laptop $100 Diagnosis and Repair service, whereby you pay just $100 + parts for repair, with free return shipping. If, after receiving their parts estimate, you decide against the repair, you pay just $30 for their diagnosis + return shipping cost.
This is a bit long so please bear with me… I have a five-year-old Dell Inspiron 7370. The graphics driver is updated. The problem I’ll describe began occurring a year ago intermittently and used to resolve on its own after a few minutes but now it is constant. When the internal battery is connected, whether or not the laptop is on AC power, the screen flashes to the extent that the laptop is unusable. The flashing occurs when rebooting when the Dell logo appears and when in BIOS setup, both of which are outside of Windows.  Once in Windows, if I adjust the brightness, the flashing changes randomly from dark to light flashing but not in a way that corresponds to the brightness setting. I tried a new battery, and nothing changed. (Old battery was two years old.) Possible a clue of some sort is that If I remove the battery and run on AC power, the screen will stay stable and usable provided the brightness level is set between 29 and 40 (Range is 0 – 99). If I go above or below that range, the flashing returns. However, while on AC power with the battery connected, there is no setting that keeps the display stable; it flashes all the time. I realize this is convoluted so I’ll try to say it another way… I'm using the laptop now with the battery disconnected. If I keep the brightness within a certain range, the display is fine but if I make the brightness lower or higher, it goes dark and flashes. If I reconnect the battery, with or without the AC power plugged in, the display flashes very dark or very bright no matter what I set the brightness to. Any thoughts will be appreciated. Thanks.
Inspiron 7370 display unstable and flashing