testing: only report subtest races once

Before this CL the code would record the number of race detector
errors seen before starting a test, and then report an error
if there were more race detector errors after the test completed.
That approach did not work well for subtests or parallel tests.
Race detector errors could be reported multiple times at each
level of subtest, and parallel tests could accidentally drop
race detector errors.

Instead, report each race detector error at most once, associated
with whatever test noticed the new error. This is still imperfect,
as it may report race detector errors for the wrong parallel test.
But it shouldn't drop any errors entirely, and it shouldn't report
any errors more than once.

Fixes #60083

Change-Id: Ic9afea5c692b6553896757766f631cd0e86192ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/494057
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>

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