runtime: ensure no GC is running in TestParallelRWMutexReaders

Currently this test makes it clear that it's unsafe for a GC to run,
otherwise a deadlock could occur, so it calls SetGCPercent(-1). However,
a GC may be actively in progress, and SetGCPercent is not going to end
any in-progress GC. Call runtime.GC to block until at least the current
GC is over.

Updates #49680.

Change-Id: Ibdc7d378e8cf7e05270910e92effcad8c6874e59
Reviewed-on: https://go-review.googlesource.com/c/go/+/366534


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>