[release-branch.go1.9] testing: don't fail all tests after racy test failure

The code was adding race.Errors to t.raceErrors before checking
Failed, but Failed was using t.raceErrors+race.Errors. We don't want
to change Failed, since that would affect tests themselves, so modify
the harness to not unnecessarily change t.raceErrors.

Updates #19851
Fixes #21338

Change-Id: I483f27c68c340928f1cbdef160abc0a5716efb5d
Reviewed-on: https://go-review.googlesource.com/57151


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>