[asan] De-flake one test.

This change replaces an in-test timeout with an unconditional blocking wait.
It speeds up normal execution significantly at the cost of hanging up indefinitely
in case of a failure. This is a very specific regression test and we don't
expect any failures in the future.

Another approach ould be increasing the timeout to ~8 seconds, which seems too
much for a lit test.

llvm-svn: 217870