ovs_assert, tests: Support NDEBUG.

ovs_assert, tests: Support NDEBUG.

./configure accepts --enable-ndebug option.  Make ovs_assert() honor
it, and make sure all test programs disable it.

The order of include files in test programs is also made uniform:

1. #include <config.h>
2. #undef NDEBUG
3. Include file of the test subject (to make sure it itself has
   sufficient include directives).
4. System includes in alphapetical order.
5. OVS includes in aplhapetical order.

Suggested-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>