ocaml: Standardize the test sequence for all bindings; implement this for OCaml.

The idea behind this change is to have a consistent set of
tests across all bindings, while at the same time saving time.
For background see:

https://www.redhat.com/archives/libguestfs/2013-April/thread.html#00069
perl: Almost standardize the Perl tests.

This needs more work, but brings them mostly up to the same
standard form as the OCaml tests and the test documentation.