(maint) Properly mock test

The Module.unmet_dependencies method will call
Environment.module_by_forge_name and will blindly rescue all exceptions.
During the test, the module's environment instance variable was nil, so
we were calling Nil.module_by_forge_name, rescuing the error and
treating it as though we didn't find the module dependency.

This commit updates the test to specify the mocked environment.

We should seriously consider removing the blind rescue all exceptions,
and at least log when they occur.