Fix import statement in unit test

[brawer]

Changed to a relative import, which it actually is. Before this change,
Python setuptools was unable to run glyphsLib unit tests on Alpine Linux.
(Alpine seems to be more strict on Python import paths?) After this change,
the unit tests get executed. They still fail, but that is another problem.

Here’s the traceback of the error before this change:

run_various_tests_on_various_files (unittest.loader._FailedTest) ... ERROR

======================================================================
ERROR: run_various_tests_on_various_files (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: run_various_tests_on_various_files
Traceback (most recent call last):
  File "/usr/lib/python3.8/unittest/loader.py", line 154, in loadTestsFromName
    module = __import__(module_name)
  File "/home/builder/aports/testing/py3-glyphslib/src/glyphsLib-5.1.7/tests/run_various_tests_on_various_files.py", line 25, in <module>
    import test_helpers
ModuleNotFoundError: No module named 'test_helpers'