Create test to check that expected migration versions are correct

[markphelps]

Re: #700

1.6.0 release was unsuccessful because we forgot to bump the expected db migration versions here: https://github.com/markphelps/flipt/pull/700/files#diff-90c8434c840e0b93eed0f28e4d501b0f48b3ac122c555178a6e963dc7d8ff311L17-L21 when new migrations were added in #685

We should automate this check, perhaps just making it a regular go test in the migrator package that will fail if there are new migrations added that have not been reflected in the above map.