Add better assertions for comparing integers.

Adds DISSECTOR_ASSERT_CMPINT() family of assertions which show the value
of each side of the comparison upon failure.

Change-Id: Ie6f2ba46d62fc864c08eb432c403fab8f1932d91
Reviewed-on: https://code.wireshark.org/review/3103
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Reviewed-by: Evan Huus <eapache@gmail.com>