sync: move lock linearity test and treat it like a performance test
This change moves test/locklinear.go into the sync package tests, and
adds a bit of infrastructure since there are other linearity-checking
tests that could benefit from it too. This infrastructure is also
different than what test/locklinear.go does: instead of trying really
hard to get at least one success, we instead treat this like a
performance test and look for a significant difference via a t-test.
This makes the methodology behind the tests more rigorous, and should
reduce flakiness as transient noise should produce an insignificant
result. A follow-up CL does more to make these tests even more robust.
For #32986.
Change-Id: I408c5f643962b70ea708930edb4ac9df1c6123ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/411396 Reviewed-by: Michael Pratt <mpratt@google.com>