tests that modify a single byte of signature or payload can fail intermittently

[fxamacker]

These two tests use an XOR that causes them to fail intermittently.

• test_cose_sign_verify_tampered_signature
• test_cose_sign_verify_modified_payload

https://github.com/mozilla-services/go-cose/blob/1ecf8f01ead363c662f6dae2683254df184a78e2/sign_verify_cose_rust_cli_test.go#L58-L65

This can be fixed by making the target byte XOR with 1, instead of itself.