Increase coverage

[weikengchen]

This PR includes some code optimization and additional tests that increase the coverage.

There could be some chances to increase the coverage further---it might have something to do with the script language.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.69231% with 11 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/utils/bitcoin_script.rs	100.00% <ø> (ø)
src/utils/mod.rs	96.22% <100.00%> (-0.07%)	:arrow_down:
src/channel/bitcoin_script.rs	44.11% <0.00%> (+1.69%)	:arrow_up:
src/channel/mod.rs	50.00% <56.25%> (+5.81%)	:arrow_up:

... and 3 files with indirect coverage changes

[weikengchen]

A next step is to improve the husky by also checking the tests during clippy.