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/shell.nix

@@ -0,0 +1,37 @@
+{ pkgs ? import (builtins.fetchTarball {
+      name = "nixos-20.09-2021-01-15";
+      url = "https://github.com/nixos/nixpkgs/archive/cd63096d6d887d689543a0b97743d28995bc9bc3.tar.gz";
+      sha256 = "1wg61h4gndm3vcprdcg7rc4s1v3jkm5xd7lw8r2f67w502y94gcy";
+    }) {} }:
+let
+   environconfig = pkgs.python38Packages.buildPythonPackage rec {
+     pname = "environconfig";
+     version = "1.7.0";
+   
+     src = pkgs.python38Packages.fetchPypi {
+       inherit pname version;
+       sha256 = "087amqnqsx7d816adszd1424kma1kx9lfnzffr140wvy7a50vi86";
+     };
+       meta = {
+         homepage = "https://github.com/buguroo/environconfig";
+         description = "Environment variables made easy";
+       };
+   };
+
+   pythonDependencies = [
+     pkgs.python38Packages.behave
+     pkgs.python38Packages.requests
+     environconfig
+   ];
+
+   nodeDependencies = (pkgs.callPackage ./node-dependencies.nix {});
+in
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.python38
+    pythonDependencies
+    pkgs.gnumake
+    pkgs.which
+    nodeDependencies.gherkin-lint
+  ];
+}