Run tests directly from the Makefile

Depends on: https://github.com/alphagov/notifications-aws/pull/905

Previously this would print some custom text with each step, and
as optionally loading a virtual environment. This moves the actual
test commands to the Makefile. While this no longer prints custom
text, it does print the command that was run:

Before (skipping other output):

  ./scripts/run_tests.sh
  Code style check passed
  Import order check passed
  ...
  JavaScript tests have passed
  ...
  Unit tests have passed

After (skipping other output):

  flake8 .
  isort --check-only ./app ./tests
  npm test
  ...
  py.test -n auto --maxfail=10 tests/
  ...

I think it's more useful to see the command being run, rather than
having to wait until it succeeds to know what was happening. Having
the command also makes it easier to run it again if it fails, rather
than having to go and find it in a script.

Makefile

@@ -49,7 +49,10 @@ generate-version-file: ## Generates the app version file
 
 .PHONY: test
 test: ## Run tests
-	./scripts/run_tests.sh
+	flake8 .
+	isort --check-only ./app ./tests
+	npm test
+	py.test -n auto --maxfail=10 tests/
 
 .PHONY: fix-imports
 fix-imports: