[travis] Build and run the unit test suite

Signed-off-by: Michael Brown <mcb30@ipxe.org>

.travis.yml

   - make -C src bin-x86_64-linux/blib.a
   - make -C src bin-x86_64-linux/tap.linux
   - make -C src bin-x86_64-linux/af_packet.linux
+  - make -C src bin-x86_64-linux/tests.linux
+  - ./src/bin-x86_64-linux/tests.linux