[Sanitizers] Fix test case that doesn't clean up after itself

Commit https://reviews.llvm.org/rG144e57fc9535 added this test
case that creates message queues but does not remove them. The
message queues subsequently build up on the machine until the
system wide limit is reached. This has caused failures for a
number of bots running on a couple of big PPC machines.

This patch just adds the missing cleanup.