[test] fix tests when executed in an apycot environment ie. when tests are executed from sources towards an installed apycot. The fix consist mainly in "moving" the input_path function from testutils.py (which is part of the installed modules) into unittest_checkers.py so we are sure we look for test data files relatively to the test directory (and not the location of the installation).

usr/share/narval/checkers
usr/share/narval/preprocessors
usr/share/narval/data
usr/lib/python*