This allows us to add `enable_test` anywhere in a build target,
without having to worry about the order in which they are included.

Previously, there was a bug that occured when adding 'mruby-test' gem
to dependencies before additional gems.

Instead of adding the 'mruby-test' gem dependency manually to a test build,
we now only need to call `enable_test` in the target. This also allows us to
call `test_enabled?` downstream when running mruby tests ourselves.

/cc #2924