[electrum] automate test discovery

Summary:
Remove the need to maintain a list of tests in the various __main__.py files by adding a run_tests.py script to the top level and using unittest.defaultTestLoader.discover.

This could alternatively be done by using pytest, tox or nose, but I would rather avoid a third-party dependency if it can be done in 15 lines of code with standard python libraries.

Test Plan: python electrum/run_tests.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14124