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>

.github/workflows/test_and_release.yml

@@ -40,7 +40,7 @@ jobs:
         docker build . -t bbvalabsci/kapow-spec-test-suite:latest
     - name: Spec test
       run: |
-        docker run --mount type=bind,source=$(pwd)/build/kapow,target=/usr/local/bin/kapow  bbvalabsci/kapow-spec-test-suite:latest behave --tags=~@skip
+        docker run --mount type=bind,source=$(pwd)/build/kapow,target=/usr/bin/kapow  bbvalabsci/kapow-spec-test-suite:latest "behave --tags=~@skip"
   doc-test:
     runs-on: ubuntu-20.04
     steps: