ofproto-dpif: Break out MPLS controller tests into their own test

This makes maintaining the controller test slightly easier
by splitting it in two.

Based on a similar patch by Joe Stringer.

Cc: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>