test(spec): Control Server API secured via cross-pinning mTLS

. We are now leveraging nix for portable dependency handling.
. There are now three types of tests: client, server and end-to-end.
. server tests exercise the actual kapow server being tested, while the
requests are performed using the test steps.
. client tests exercise the actual kapow client being tested, while the
requests are served using the test steps.
. e2e test exercise the actual kapow program in its dual role of client
and server (¡como tiene que ser!).

Co-authored-by: Roberto Abdelkader Martínez Pérez <robertomartinezp@gmail.com>

spec/test/features/control/list/success.feature

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+@server
 Feature: Listing routes in a Kapow! server.
   Listing routes allows users to know what URLs are
   available on a Kapow! server. The List endpoint returns