testdata: attempt to deflake timers/goroutines tests

[jakebailey]

Fixes #4963
Fixes #4965

I got tired of hitting these flakes locally and in CI. These changes seem to make them pass, but it's a little unclear without more eyes whether or not I have defeated the purpose of these tests somehow as I of course arrived here after these were committed...

[deadprogram]

I also find those failures very annoying. The tests are not so precise as to preclude these changes. Anyone have any objections for me to squash/merge?

[aykevl]

This kinda does defeat the original purpose of these tests. I wrote them a long time ago to show that TinyGo worked concurrently.

What about only running this test on systems without thread support? I don't think they're flaky there.

[deadprogram]

I think perhaps now that I have increased the runner sizes this should be less of a problem for the CI.

[jakebailey]

I hit this frequently when doing local testing, so I don't think increasing a builder size really fixes it...

[jakebailey]

What about only running this test on systems without thread support? I don't think they're flaky there.

Pushed that, at least

[deadprogram]

@jakebailey can you please rebase against the latest dev to rerun the tests? Thank you.