Allow `gazelle_test` to print out the `bazel run` command that would fix the test error

[michael-pplx]

Currently when the test fails it just prints out the diff. For people working in a code base that is being newly introduced to Bazel, it may not be clear what one needs to do to resolve the error. If the command can be automatically be determined like write_source_files then great, but if not, it would be good enough to have gazelle_test accept an argument for a custom failure message to print out.