runtime/pprof: deflake TestGoroutineCounts

runtime/pprof: deflake TestGoroutineCounts

TestGoroutineCounts was flaky when running on a system under load.
This happened on three builds the last couple of days.

Fix this by running this test with a single operating system thread, so
we do not depend on the operating system scheduler.  50 000 tests ran
without failure with the new version, the old version failed 0.5% of the
time.

Fixes #15156.

Change-Id: I1e5a18d0fef4f72cc9a56e376822b2849cdb0f8b
Reviewed-on: https://go-review.googlesource.com/43590
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>