This PR adds missing unit test coverage for the Python ECDSA implementation, which should be useful for detecting potential problems early whenever changes in the test framework's Python implementation of secp256k1 are made (e.g. #26222). Note that right now we don't call ECPubKey.verify_ecdsa anywhere in our tests, so we wouldn't notice if it is broken at some point.
To keep it simple, the already existing unit test for Schnorr signatures is extended to also check ECDSA signatures. For that purpose, the dictionary storing private-key/public-key entries use their legacy types ECKey/ECPubKey instead of bare byte-arrays, and for Schnorr signing/verification the necessary conversions (ECKey -> bare private key, ECPubKey -> x-only pubkey) is done later when needed. To avoid code duplication, a helper function random_bitflip for damaging signatures is introduced.
The unit test can be run by either calling it for this single module:
$ python3 -m unittest ./test/functional/test_framework/key.py
or simply running $ ./test/functional/test_runner.py which calls all test framework module's unit tests at the start (see TEST_FRAMEWORK_MODULES list).
