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>

Makefile

@@ -42,8 +42,8 @@ coverage: test race
 install: build
 	CGO_ENABLED=0 $(GOINSTALL) ./...
 
-acceptance: install
-	make -C ./spec/test
+acceptance: build
+	cd ./spec/test && PATH=$(PWD)/build:$$PATH  nix-shell --command make
 
 deps:
 	@echo "deps here"