[electrum] let CoinChooser.make_tx take a list of TxInput

Summary:
Move the dict -> TxInput conversion out of coinchooser.py

Add basic test coverage

Depends on D14560

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14561