project tests: DummyExecutor for MSYS2 and OpenBSD

Added and tested on MSYS2/MinGW which doesn't implement the required
semaphore locks in the multiprocessing module:

Traceback (most recent call last):
  File "C:/msys64/mingw64/lib/python3.5\multiprocessing\synchronize.py", line 29, in <module>
    from _multiprocessing import SemLock, sem_unlink
ImportError: cannot import name 'sem_unlink'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "run_project_tests.py", line 560, in <module>
    (passing_tests, failing_tests, skipped_tests) = run_tests(all_tests, 'meson-test-run', options.extra_args)
  File "run_project_tests.py", line 406, in run_tests
    executor = conc.ProcessPoolExecutor(max_workers=num_workers)
  File "F:/msys64/mingw64/lib/python3.5\concurrent\futures\process.py", line 390, in __init__
    EXTRA_QUEUED_CALLS)
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\context.py", line 101, in Queue
    return Queue(maxsize, ctx=self.get_context())
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\queues.py", line 42, in __init__
    self._rlock = ctx.Lock()
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\context.py", line 65, in Lock
    from .synchronize import Lock
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\synchronize.py", line 34, in <module>
    " function, see issue 3770.")
ImportError: This platform lacks a functioning sem_open implementation, therefore, the required synchronization primitives needed will not function, see issue 3770.

See also:
https://bugs.python.org/issue3770
https://github.com/mesonbuild/meson/issues/1323

According to 3770, the same problem also exists on OpenBSD, so this
will potentially also be useful there.

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