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/steps/get_environment.py

@@ -0,0 +1,9 @@
+#!/usr/bin/env python
+
+import json
+import os
+import sys
+
+if __name__ == '__main__':
+    with open(os.environ['SPECTEST_FIFO'], 'w') as fifo:
+        json.dump(dict(os.environ), fifo)