docs/devel/testing.rst: add missing newlines after code block

docs/devel/testing.rst: add missing newlines after code block

The line immediate following a ".. code::" block is considered
to contains arguments to the "code directive".  The lack of a
new line gives me during at parse time:

   testing.rst:63: (ERROR/3) Error in "code" directive:
   maximum 1 argument(s) allowed, 3 supplied.

   .. code::
     make check-unit V=1

   testing.rst:120: (ERROR/3) Error in "code" directive:
   maximum 1 argument(s) allowed, 3 supplied.

   .. code::
     make check-qtest V=1

Let's add the missing newlines, both for consistency and to
avoid the parsing errors.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20181004161852.11673-6-crosa@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>