[electrum] rename tests in test_bip21

Summary:
No need to repeat parse_URI in every test name and helper function name, just put it in the class name.

Depends on D15234

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15235