Also add a specific test that fails adaptor sig deserialization because with the
correct size buffer that's not guaranteed anymore with the existing test.
Previously the ECDSA signature had an overflowing s value, which after the sync
with upstream results in a failing VERIFY_CHECK in the inversion function.
However, normally parsed signatures shouldn't contain overflowing s values.
This commit adds test coverage including Cirrus scripts, Valgrind
constant time tests for secret data, API tests, nonce function tests,
and test vectors from the spec.
