Moved last three GCS test files over to new testcase class.

These files were created before I made the new testcase base class:
 * test_resumable_downloads.py
 * test_resumable_uploads.py
 * test_connection.py

There were many places in these files where buckets, keys, and on-disk files were being created but not securely cleaned up. I converted all of these to use the centralized functions in GSTestCase to ensure that tests always clean up after themselves. It also made some of the tests run a lot faster because I removed unnecessary shared variables created in setUp/tearDown that weren't being used in all tests.