Update 'running threaded' messages after option parsing change

"Running multithreaded with n threads (from LLDB_TEST_THREADS)" is
incorrect in the common case, because the thread count is now obtained
from a --threads option, or a default of multiprocessing.cpu_count().

I left the messages in for now as there are some intermittent issues
that happen while running tests multithreaded and it may help in
triaging those.  We may wish to just remove them later on.