tests: Really move command from docker-compose.yml to Makefile

This makes it easier to understand the interaction between the outer and
inner make, as one doesn't need to dig through docker-compose.yml and
possibly Dockerfile to find what command will be run.

It does break the ability to run `docker-compose run --rm tests` but
that's a good thing I believe: I only ever use that when I need a shell
in the container for debugging purposes.

Makefile

@@ -3,7 +3,7 @@ export LC_ALL := en_US.UTF-8
 PYTHON ?= python3
 
 test:
-	docker-compose run --rm tests
+	docker-compose run --rm tests make xvfb-cover-pytest
 
 pytest:
 	$(PYTHON) -m pytest -vv $(PYTEST_FLAGS) tests/

docker-compose.yml

@@ -8,7 +8,6 @@ services:
     security_opt:
       - label=type:container_runtime_t
     tty: true
-    command: make xvfb-cover-pytest
     environment:
       PYTEST_FLAGS: "-o cache_dir=/tmp/pytest-cache ${PYTEST_FLAGS:-}"
       TRAVIS: