* Remove runtest_ns(): pass directly ns to runtest().
* Create also Regrtest.rerun_failed_tests() method.
* Inline again Regrtest.run_test(): it's no more justified to have a method
Move the code to run tests in multiple processes using threading and subprocess
to a new submodule.
Move also slave_runner() (renamed to run_tests_slave()) and
run_test_in_subprocess() (renamed to run_tests_in_subprocess()) there.