Client assertion signing is algorithm-agnostic, but the fixtures only
produced RS256 keys, so nothing proved it. Covers RSA, ECDSA and Ed25519,
including both RFC 9864 identifiers.
Also drops a kty assignment that jwx ignores and that would have mislabelled
non-RSA keys had it taken effect.
Satisfies errcheck. Errors that carry no actionable information are
ignored explicitly: writes to an already-committed response, and closing
a fully read response body or file.
Test fixture setup asserts with require.NoError instead, since a failure
there means the fixture itself is broken.
Trong Huu Nguyen
