Fixes test that fails under bdist_egg installs.
Fixes test that fails under bdist_egg installs.

Follow-up to
https://github.com/GoogleCloudPlatform/gsutil/commit/19b55f9b7bb8c8daa1124ee17cb72a932475357e.
During review, I missed that we should resolve test_data files using
pkgutil, and not with a literal path.  For egg installations, the
GSLIB_DIR var points to a Python egg (which is not a directory).

GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil