CONTRIBUTING.md: Strengthen testing recommendataions.

CONTRIBUTING.md: Strengthen testing recommendataions.

Patches that modify existing code can break expected behaviour.
Flag this by testing the patch with 'make check' prior to submission.

Furthermore, it is not sufficient to only test patches that add files
using 'make distcheck'; the compile flags for this target could change
the definition of some functions (ovs_assert, for example), altering
the outcome of some unit tests. Rather, it is preferable to use a
combination of 'make distcheck' with 'make check' to cover all bases.

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>