Split most HTools test code into separate files

Except for Ganeti.HTools.JSON, which needs rename, we split all the
other test suites into separate files.

We have to add another common test helper, due to import dependencies
(sigh), but otherwise this split is pretty straightforward.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>