Factor-out the driver to run_nxdtest in a separate thread done in
0ad45a9c (Detect if a test leaks processes and terminate them) from
test_run_procleak into run_nxdtest, so that all tests are run this way
for pytest.timeout, if requested, to work universally for all tests.

The logic of how we run nxdtest under tests and handle timeout will be,
hopefully, soon reworked more, but it is anyway good to keep this logic
in only one place.

/reviewed-by @jerome
/reviewed-on !14