[electrum] move the trivial RequestHandler out of the test case

Summary:
This makes the TrivialRequestHandler available for other tests. Move-only, no change in behavior.

Depends on D16414

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16417