This does not change the behavior of the test_runner.py script which was already discovering the tests.
It fixes running the test suites individually when the suite() function was not maintained, e.g.:
python -m electrumabc.tests.test_wallet
From the unitest.main doc:
The defaultTest argument is either the name of a single test or an iterable of test names to run if no test names are specified via argv. If not specified or None and no test names are provided via argv, all tests found in module are run.
Depends on D14442