make-release: unit tests for MwVersion

Basic unit tests for the make-release MwVersion class using python
unittest. Will let us refactor MwVersion to change 'branch' to 'tags'
and populate the real branch name (aka: REL#_## or master).

$ python -m unittest discover --verbose
test_incomplete_version (tests.test_mwversion.TestMwVersion) ... ok
test_major_version (tests.test_mwversion.TestMwVersion) ... ok
test_master (tests.test_mwversion.TestMwVersion) ... ok
test_minor_version (tests.test_mwversion.TestMwVersion) ... ok
test_release_candidate (tests.test_mwversion.TestMwVersion) ... ok
test_release_candidate_bumps (tests.test_mwversion.TestMwVersion) ... ok

----------------------------------------------------------------------
Ran 6 tests in 0.002s

OK
$

The runtest.sh is a helper to let people easily run the tests without
having to figure out the exact command.

bug: 57088
Change-Id: I25f5a9d28e4e1c0bb73ba3603f741d8be2c05f0b