evidence: improve unit tests

[tessr]

As part of our "evidence hardening" push, we should unit test the evidence reactor more thoroughly. Although the coverage numbers aren't terrible, it's mostly happy-path testing. (See https://codecov.io/gh/tendermint/tendermint/src/master/evidence/verify.go for a good example of this.) We should increase the unit tests to cover unhappy-paths and other error cases.