[electrum] add a check-electrum target

Summary:
And run it with check-all and check-extended.
This adds about 5 seconds to the runtime of these test suites.

Depends on D14135

Test Plan:

ninja check-electrum
ninja check-all
ninja check-extended

@bot electrum-tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14126