Fixup lint testing for changes in D6798

Summary:
We have some linters that trigger based on the path name
in the tree (some rules apply in some dirs and not others).
The changes in D6798 caused all the paths to appear to be outside
the tree, so allow for passing a fake through from those test cases
that are sensitive to this.

We also have a test for the copyright linter, and that needs to read
settings from the .arcconfig file. The change to faking a working
copy meant that this config option was effectively unset, so add a way
to pass the entire arcconfig through from the tests that need it.

Lastly, the logic to skip deleted files needs to be special cased
when we're faking paths like this: if we've added data for a file
in the testable engine, we should also consider that file as existing.

Test Plan:
arc unit --everything here, and passing our tests in
our repo over there.

Reviewers: epriestley, mareksapota

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6841