secp256k1/ecdsa: Add compact signature tests.

[davecgh]

This requires #2914.

This adds tests for compact signature generation and associated public key recovery for both the positive and negative paths and reworks the tests that deal with signing random messages with random private keys and recovering the associated public key to make them more consistent with modern practices in the code as well as to expand the testing methodology to include additional assurances.

The following is a high-level overview of the changes:

• Add tests to ensure test data that was independently verified with the Sage computer algebra system produces the expected results
• Add tests which are specifically designed to exercise all of the recovery failure paths
• Adds a test that uses random data:
  • Signs random representative hashes instead of a fixed non-hashed message
  • Signs, recovers, and verifies both the compressed and uncompressed public key variants for each iteration
  • Ensures mutating a random bit in the good signature results in failure to recover the original public key
  • Ensures mutating a random bit in the message hash that was originally signed results in failure to recover the original public key from the good signature