Skip puppeteer download on unit test runs, fix windows tests, polish pipeline

[Tyriar]

Fixes #2539
Fixes #2542
Fixes #2543

[Tyriar]

Skipping puppeteer download seems to save about 15 seconds on Linux, let's see if caching yarn cache works.

[Tyriar]

• Baseline (2m58s): https://dev.azure.com/xtermjs/xterm.js/_build/results?buildId=2643
• Cache yarn cache, no puppeteer
  • Miss (3m22s, cache took 28s): https://dev.azure.com/xtermjs/xterm.js/_build/results?buildId=2647
  • Hit: (2m29s): https://dev.azure.com/xtermjs/xterm.js/_build/results?buildId=2648
• Cache node_modules (~300mb), no puppeteer
  • Miss (3m12s)
  • Hit (2m15s)

[jerch]

@Tyriar So the difference is basically a minute for a single pipeline?

[Tyriar]

@jerch yeah, integration tests are mostly unchanged at but I'll see if I can reduce that a bit too. So right now we'll get faster compile/unit test feedback.

[Tyriar]

Caching node_modules for integration tests didn't seem to work, also it ended up as 1gb so it was going to be slower anyway if it did.

[Tyriar]

Final results gives faster compile and unit test builds to let us iterate faster on PRs:

	Before	After
Linux	2m 58s	1m 42s
macOS	1m 58s	1m 50s*
Windows	3m 23s**	2m 3s

* The actual run took 2m 23s but that's because cache restore took an abnormally long time just this run, number was adjusted for typical runtime
** This is the approx value, Windows wasn't running tests before so it's the actual number 2m 55s plus 28s for unit tests

Integration tests are basically the same.

[Tyriar]

This passed fully on a rebuild