Before this change backend integration tests depended on each other,
so tests could not be retried.
After this change we nest tests to ensure that tests are provided with
the starting state they expect.
Tell the integration test runner that it can retry backend tests also.
This also includes bin/test_independence.go which runs each test
individually for a backend to prove that they are independent.
Add a new flag to the drive backend to allow document conversions oni upload.
The existing --drive-formats flag has been renamed to --drive-export-formats.
The old flag is still working to be backward compatible.
Make use of the mime package to find matching extensions and mime types.
For simplicity, all extensions are now prefixed with "." to match the
mime package requirements.
Parsed extensions get converted if needed.
