import-checker: fix test to make a real package

Otherwise "testpackage" wouldn't be counted as a package when building a
list of imported symbols.

tests/test-check-module-imports.t

@@ -11,6 +11,7 b" it's working correctly."
 Run additional tests for the import checker
 
   $ mkdir testpackage
+  $ touch testpackage/__init__.py
 
   $ cat > testpackage/multiple.py << EOF
   > from __future__ import absolute_import