[electrum] exclude the regtest directory from the unit test discovery

Summary:
In a future diff I want to replace the pytest framework with the regular the unittest module for functional tests (electrumabc/tests/regtest/), but we don't want these functional tests to be found and executed by the unit test runner. They will need to run with their own framework, with a node compiled...etc

Add a simple filter class to skips all tests that contain "regtest" in their fully qualified module name (e.g electrumabc.tests.regtest.test_rpc_misc)

Test Plan:
Rebase on D18293 and try:
cd electrum && python test_runner.py

See that it doesn't fail because of the functional tests not finding bitcoind.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18295