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/Dockerfile

@@ -1,16 +1,18 @@
-FROM python:3.7-alpine
+FROM nixos/nix:2.3.6
 
 # Install CircleCI requirements for base images
 #   https://circleci.com/docs/2.0/custom-images/
-RUN apk upgrade --update-cache \
- && apk add git openssh-server tar gzip ca-certificates
+# RUN apk upgrade --update-cache \
+#  && apk add git openssh-server tar gzip ca-certificates
 
 # Install Kapow! Spec Test Suite
 RUN mkdir -p /usr/src/ksts
 WORKDIR /usr/src/ksts
 COPY features /usr/src/ksts/features
-COPY Pipfile Pipfile.lock /usr/src/ksts/
-RUN pip install --upgrade pip \
- && pip install pipenv \
- && pipenv install --deploy --system \
- && rm -f Pipfile Pipfile.lock
+# COPY Pipfile Pipfile.lock /usr/src/ksts/
+# RUN pip install --upgrade pip \
+#  && pip install pipenv \
+#  && pipenv install --deploy --system \
+#  && rm -f Pipfile Pipfile.lock
+COPY ./*.nix ./
+ENTRYPOINT [ "nix-shell", "--command" ]