test: import build_container_fixture as _

We need to import the fixtures and use them indirectly, which makes it
appear like an unused import.  Instead of disabling the linter on the
line, let's suppress the warning by using _ as the import name.

I find this nicer.

Author: achilleas-k

test/test_manifest.py

@@ -9,7 +9,7 @@
 import pytest
 
 import testutil
-from containerbuild import build_container_fixture  # pylint: disable=unused-import
+from containerbuild import build_container_fixture as _
 from containerbuild import make_container
 from testcases import gen_testcases