Ensure screenshots are taken and uploaded to Skia gold, or fail the test

[matanlurey]

From Discord:

Running into some weirdness.

In flutter/engine#50583, I added another set of tests running Android API v28, expecting new goldens. I only saw a few (<5) and assumed it just matched existing digests and moved on.

Now on an entirely unrelated PR, https://flutter-engine-gold.skia.org/cl/github/50601, they are showing up.

This is pretty crucial because these are the only rendering tests for the Android embedder, if I can't rely on the goldens all of this effort might be wasted.

I'll file a separate issue for some possible race-y conditions that result in a screenshot never being taken, but the test should definitely fail if an expected screenshot is not created and uploaded. @gaaclarke had some similar work for the Impeller golden files, and ended up with a diff file approach that seems reasonable to copy or at least be inspired by.

The outcome of this issue would be:

• We're sure that every test is taking and uploading a screenshot
• A missing screenshot (or failed upload) fails the test suite
• In combination with Filter the ADB logcat messages to the running application/test suite #143458, logging makes it clear what did/did not happen

[gaaclarke]

I know i just spent time writing up the functionality I needed in the python. If you need it though it might be worth transitioning it to a dart script that can be used in multiple places

$ dart compare_dir_contents.dart <path to text file> <path to dir>

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