[test] Test verification of constructed signature

[test] Test verification of constructed signature

Some signature schemes (such as ECDSA) allow for non-deterministic
signatures.  Provide more information in test results by performing
verification of the constructed signature even when it does not match
the expected test case result: this allows us to distinguish between a
bug that is generating invalid signatures and a bug that is generating
valid but non-canonical signatures.

Signed-off-by: Michael Brown <mcb30@ipxe.org>