ofp-print: Add tests for OpenFlow message formatting.

ofp-print: Add tests for OpenFlow message formatting.

It's become clear over the past few weeks that some people really depend
on the formatting of ofp-print output.  It has also become clear that OVS
doesn't have anything that actually checks or enforces the formatting of
the output.  The former is not something I want to encourage, but the
latter is somewhat worrisome because the formatting does change from time
to time and I want to make sure that that is intentional.

So this commit adds at least one test for each supported OpenFlow and
Nicira extension message, so that we can better keep track of changes.