Ensure that `file=` will actually work when passing into make test

- Fixes #48
- Support both `FILE=` and `file=`

Makefile

@@ -41,4 +41,4 @@ migrate: ## Migrate the wagtail bakery site migrations
 	docker-compose exec web python manage.py migrate
 
 test: ## Run all wagtail tests or pass in a file with `make test file=wagtail.admin.tests.test_name.py`
-	docker-compose exec -w /code/wagtail web python runtests.py $(FILE)
+	docker-compose exec -w /code/wagtail web python runtests.py $(file) $(FILE)