This test is slightly flaky on the s390x builder and I suspect that
the 100ms timeout is a little too optimistic when the VM is starved.
Increase the timeout to 5s to match the other part of the test.

Fixes #26231.

Change-Id: Ia6572035fb3efb98749f2c37527d250a4c779477
Reviewed-on: https://go-review.googlesource.com/122315
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>