Adjust ThreadPoolExecutorTest#testPoolSizeInvariants

Commit dfec9b5386ca028cc1468f3e2717120ab6274702 alters how
ThreadPoolExecutor#setCorePoolSize behaves. This change
fixes #testPoolSizeInvariants to take it into account.

Bug: 27702221
Change-Id: If294e734136dfca39c047776b925c4f8d49a1ec2
1 file changed