test: tweak test to avoid unpreemptible loop with gccgo
test: tweak test to avoid unpreemptible loop with gccgo

This test contains a very tight loop with locking/unlocking that can
wind up as an unpreemptible when compiled with gccgo, depending on
inlining. Tweak the test slightly to avoid this problem.

Change-Id: I155fd2b4bfea961244eb6c6594c24ab03d32d41c
Reviewed-on: https://go-review.googlesource.com/c/go/+/193619
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,CAFxX/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,golang/go,nawawi/go,akutz/go,cloudflare/go,akutz/go,golang/go,nawawi/go,golang/go,CAFxX/go,akutz/go,nawawi/go,golang/go