[electrum] add command line option to test the update_checker

Summary: Previously the only way to test that everything worked as expected (correct signature, proper JSON format...) with the version checker was to update the JSON file and push to the main repository. This adds a --test-release-notification command line argument to electrum-abc to test it locally, without relying on the GitHub url to already work.

Test Plan:
Run ./electrum-abc -v --test-release-notification
Modify the release file in various ways and repeat, to test for wrong signatures, wrong JSON format.

Also check without the --test-release-notification option, first without touching the code, then after downgrading the version below 5.2.1 in version.py, to check the remote URL still works as expected.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14147