runtime: deflake TestGCTestIsReachable

This is a simple workaround for a bug where runtime.GC() can return
before finishing a full sweep, causing gcTestIsReachable to throw. The
right thing is to fix runtime.GC(), but this should get this test
passing reliably in the meantime.

Updates #45315.

Change-Id: Iae141e6dbb26a9c2649497c1feedd4aaeaf540c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/307809


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>