Remove white-box testing

[elbrujohalcon]

Since #30 we have a test suite (wildcard_SUITE) that calls a function that should not be exported at all: xref_runner:find_dirs/1.

That function was intended to be private and it's only exported to be used by the test. That's not right. If the function is not used by production code, it shouldn't be exported.

We need to redefine the tests in order to actually execute/cover all the paths relative to the function without actually calling the function itself.

And, now that we are on it, we should as well rename that SUITE (if we're not removing it) with the proper prefix: xref_wildcard_SUITE