Add functional tests, fix unset exit codes on panics

[JohnStarich]

I ran into a few panics in my projects and realized those triggered package failures but were "passing" the tests because they exited with 0.

This PR fixes those exit codes so it doesn't miss anything.

I had difficulty reproducing the failures for a while, which necessitated this new functional test style. I hope it is to your liking! 😄

• Refactor to support functional tests
• Fix unset exit code from uncaught exception
• Fix unset exit code for panic in next tick of event loop

[JohnStarich]

Bummer. Windows tests are failing with the same issue as #40

[JohnStarich]

I believe we can fix the Windows issue with this: #40 (comment)
It's not great, but it does work. Hopefully we can resolve that issue.

[agnivade]

I haven't been able to find time to get to review this. It's on my list, thank you for your patience!

[JohnStarich]

@agnivade Looks like cleanenv worked! I also had to bump the test timeout because Windows is slower.

We're all 🟢 now!

[agnivade]

Sorry for the delay!

[agnivade]

Minor cleanup comments. Looking great overall :)

[JohnStarich]

Thanks for the reviews!
That Windows 1.19 test run is constantly flaking. I wonder if it’s getting overwhelmed somehow. Would you mind giving it a restart, as I don’t have access?

[agnivade]

I have a feeling this is going to remain flaky. I have restarted for now, but if this continues, we might want to skip the test in Windows.

[JohnStarich]

Thank you for the merge! Would you mind cutting a new release when you have a chance?

That's fair. It looked to me like it wasn't always the same test that failed on Windows – I'm not certain though. It's possible too many tests were running in parallel, so we might consider go test -parallel N.

[agnivade]

Yeah, good point on the parallelism. I think we might be going a bit overboard on that. I am leaning towards simply removing t.Parallel to keep things simple.

[agnivade]

Btw, I've cut a new release.