Add a NEWS entry about the cmdlib test framework and increased test
coverage, and mention that at least python-mock 1.0.1 is required as of
Ganeti 2.10.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>