Rework parallel test process count logic

The default value for opt.thread_count was multiprocessing.cpu_count(),
which meant the LLDB_TEST_THREADS environment variable was never used.
It's not easy to pass the -t option to the test run when invoking it
from e.g. 'ninja check-lldb', so having the environment variable as an
option is useful.

Change the logic so that the thread count is set by the first one of:

  1. The -t option to test/dosep.py
  2. The LLDB_TEST_THREADS environment variable
  3. The machine's CPU count from multiprocessing.cpu_count()