runtime: "fix" non-preemptible loop in TestParallelRWMutexReaders
runtime: "fix" non-preemptible loop in TestParallelRWMutexReaders

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 <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,Cofyc/go,codestation/go,golang/go,CAFxX/go,codestation/go,cloudflare/go,codestation/go,cloudflare/go,josharian/go,codestation/go,nawawi/go,akutz/go,CAFxX/go,codestation/go,cloudflare/go,golang/go,codestation/go,josharian/go,Cofyc/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,golang/go,CAFxX/go,christopher-henderson/Go,josharian/go,CAFxX/go,cloudflare/go,Cofyc/go,golang/go,Cofyc/go,josharian/go,CAFxX/go,nawawi/go,cloudflare/go,josharian/go,nawawi/go,Cofyc/go,akutz/go,CAFxX/go,codestation/go,Cofyc/go,josharian/go,Cofyc/go,codestation/go,akutz/go,josharian/go,akutz/go,golang/go,akutz/go,Cofyc/go,josharian/go,akutz/go,akutz/go,cloudflare/go