secp256k1/ecdsa: Add sign and verify tests.

[davecgh]

Although the ECDSA code has been extensively tested through usage and external fuzz testing, there are not currently any unit tests for the non-compact signing and verification path.

This resolves that by adding signing and verification tests for both the positive and negative paths.

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

• Refactors the primary logic for the regular ECDSA signing code to a separate function that accepts an arbitrary nonce to facilitate fully testing both the positive and negative paths and provides more flexibility for potentially using other nonce generation mechanisms in the future should it be necessary
• Add sign and verify tests with known good test data that is easily reproducible and has been independently verified with the Sage computer algebra system
  • The test data includes the original messages so the resulting hashes are independently verifiable and have a known source
  • The test data includes variations of signing the same data with different keys and nonces, both deterministically generated via RFC6979 and random, and signing different data with the same keys
• Add tests to ensure the aforementioned test data that was independently verified produces the expected results
• Add tests which are specifically designed to exercise conditions that lead to invalid signatures
• Add tests to help ensure the signature verification code fails to verify edge conditions as expected by crafting signatures that are specifically designed to hit them
• Add test to sign and verify random data
  • Generates random keys and messages with each run from a new random seed and log that seed in the event of failure
  • Signs the rand message with the random key and ensures the produced signature verifies correctly
  • Ensures mutating a random bit in each good signature results in that mutated signature failing to verify the original message
  • Ensures mutating a random bit in each message hash that was originally signed results in the original good signature failing to verify the new mutated message

[rstaudt2]

Nice increase in coverage!