[llvm-cov] Disable threading in a test. NFC.

PR30735 reports an issue where llvm-cov hangs with a worker thread
waiting on a condition, and the main thread waiting to join() the
workers. While this doesn't appear to be a bug in llvm-cov or the
ThreadPool implementation, it would be helpful to disable the use of
threading in the llvm-cov tests where no test coverage is added.

More context: https://bugs.llvm.org/show_bug.cgi?id=30735

llvm-svn: 307610