TestParallelRWMutexReaders has a non-preemptible loop in it that can
deadlock if GC triggers. "Fix" it like we've fixed similar tests.

Updates #10958.

Change-Id: I13618f522f5ef0c864e7171ad2f655edececacd7
Reviewed-on: https://go-review.googlesource.com/73710
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>