[electrum] move BIP21 parsing tests to their own file

Summary:
test_util contains a collection of unrelated tests, so lets move these ones out before adding more BIP21 tests.

This is a move-only commit.

Depends on D15233

Test Plan:

python test_runner.py

python -m electrumabc.tests.test_bip21

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15234