Enable tests on Mac bots

[Hixie]

Turns out we're still not actually running the tests on Mac bots.

[Hixie]

#6961 has to be fixed first

[Hixie]

#6964 too

[yjbanov]

I think it would be better if we could run them on Travis' Mac environment, which would allow us to catch regressions earlier, but last time I checked (#4027) the tests froze at random locations.

[Hixie]

Well we should run them in both places, but running them on either is better than none...

[Hixie]

Done.

[github-actions]

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