import-checker: fix test to make a real package

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.

--- a/tests/test-check-module-imports.t	Sat May 14 14:03:12 2016 +0900
+++ b/tests/test-check-module-imports.t	Sat May 14 13:49:46 2016 +0900
@@ -11,6 +11,7 @@
 Run additional tests for the import checker
 
   $ mkdir testpackage
+  $ touch testpackage/__init__.py
 
   $ cat > testpackage/multiple.py << EOF
   > from __future__ import absolute_import