Refactor keys_test.go into tabular tests
Refactor keys_test.go into tabular tests

Improves coverage of TestSignatureAlgorithm.

AlCutter/trillian,AlCutter/trillian,gtank/trillian,codingllama/trillian,Martin2112/trillian,gdbelvin/trillian,daviddrysdale/trillian,daviddrysdale/trillian,Martin2112/trillian,google/trillian,daviddrysdale/trillian,RJPercival/trillian,gdbelvin/trillian,gtank/trillian,google/trillian,RJPercival/trillian,gdbelvin/trillian,codingllama/trillian,Martin2112/trillian