[electrum] clarify the concept of number of sigs in a TxInput

Summary:
The previous num_sig property did not match the coin dict implementation nor its own documentation. The length of the signature list can be more than the required number of signatures for incompletely signed multisig inputs.

Split this into two properties:

• the number of required signatures
• the number of provided valid signatures in the signatures list (items that are not None)

The length of the signatures list including None items does not need its own property.

Add a 2-of-3 partially signed transaction test to cover the case for which len(signatures) != num_required_sigs (m != n).

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14483