XT::Files is the Tests that know what they want to check (e.g. module files),
but it's the distribution that knows where these files can be found (e.g. in
the lib directory and in the t/lib directory). Without XT::Files you may add
the same code to multiple .t files under xt that iterate over a check function
of the test. XT::Files is a standard interface that makes it easy for author
tests to ask the distribution for the kind of files it would like to test. And
it can easily be used for author tests that don't support XT::Files to have the
same set of files tested with every test.