runtime/race: deflake tests

[dvyukov]

See https://go-review.googlesource.com/#/c/18968 for context.
Race tests run with GOMAXPROCS=1, this makes them more or less reliable. But the ultimate solution is to explicitly annotate tests with required execution order by means of a special "invisible" synchronization primitive (that's what is done for C++ ThreadSanitizer tests). But that would require going over 350 tests.

[gopherbot]

CL https://golang.org/cl/18968 mentions this issue.

[rsc]

Ideally this would happen, but I think Dmitriy is the only one who can really do it.

[bradfitz]

#19604 was marked as a dup of this bug.

If these tests can't be made reliable, I think it's time to disable them. Or give them, say, 10 chances to succeed before reporting a failure.

Marking this as Go 1.9 to either fix them, disable them or make them try harder to pass.