testing: skip parallel tests when -failfast is enabled

[rubensayshi]

The t.Parallel method does not interact well with the -failfast flag.
See #30522.

With -failfast enabled tests blocking on t.Parallel() can be skipped
after the first test failure.
This won't immediately kill any parallel tests which are already running
but it will prevent more tests from starting.

I've chosen to use t.SkipNow() when we detect that we want to failfast
after a parallel test wakes up.
This to me seemed like the cleanest solution and doesn't hide
what is happening.
Though arguably we could print a more detailed message?

Fixes #30522

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Bryan Mills:

Patch Set 1: Hold+1

(1 comment)

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This PR is being closed because golang.org/cl/413236 has been abandoned.

This CL doesn't seem to work if t.Parallel is called by a goroutine.