tests: move blackbox testing of tags to test-tags.t
We're going to refactor tags cache shortly. It is easier to test the
blackbox logging if these tests are in test-tags.t.
tests: move mock blackbox extension into own file
Having all blackbox log testing in test-blackbox.t isn't scalable. Move
the mock blackbox extension into its own file so we can start to move
blackbox logging into other tests.