unit-test: Add ovstest

unit-test: Add ovstest

Changing one of the files in the Open vSwitch ``lib'' directory
causes 43 binaries to be relinked, which takes a lot of time even with
parallel ``make''.  31 of those binaries are in the ``tests''
directory.  ovs-test attemps to combine most of those binaries into a
single test program that just takes a subcommand name as its first
command-line argument.

The following patch makes use of this infrastructure.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>