[3.5] bpo-30540, bpo-30523: Add --matchfile and --list-cases options to regrtest...

[3.5] bpo-30540, bpo-30523: Add --matchfile and --list-cases options to regrtest (#2250)

* bpo-30540: regrtest: add --matchfile option

* Add a new option taking a filename to get a list of test names to
  filter tests.
* support.match_tests becomes a list.
* Modify run_unittest() to accept to match the whole test identifier,
  not just a part of a test identifier.

For example, the following command only runs test_access()
of the FileTests class of test_os:

$ ./python -m test -v -m test.test_os.FileTests.test_access test_os

* bpo-30523: regrtest: Add --list-cases option

* Add --list-cases option to regrtest
* Add get_abs_module() function, use it in list_cases()
* Add ns mandatory positional argument to runtest()
  and runtest_inner()
* Add file optional parameter to printlist()

Co-Authored-By: Louie Lu <git@louie.lu>