[electrum] add tests for BIP21 parsing and creation

Summary:
Add an op_return test case for TestParseURI and add a suite for create_URI.
The new test suite shows that currently special chars are not escaped for the op_return paramater. This will be fixed in a separate diff.

Depends on D15235

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15236