Refactor and speed up e2e tests

[bigspider]

test_e2e_multisig.py, test_e2e_miniscript.py, and test_e2e_tapscripts.py Have a lot of shared code that could be abstracted and put in common utility file.

Moreover, we can reduce a lot the running time of the tests by skipping the registration of the wallet policies (that is, computing the hmac locally instead of having that as part of the test); since we have plenty of speculos tests for that, this would reduce the running time of the suite without significantly reducing the test coverage.