test: MiniWallet: fix fee calculation for P2PK and check tx size

Summary:
Backport of core#22089.

Instead of the retry loop to get a fixed DER signature size, we could use the Schnorr signatures which are fixed size.
However the only test that uses the P2PK mode so far is the DER signature test, and using schnorr would invalidate the test.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16411