Merge pull request #1012 from cderici/makefile-tests-quality-of-life

#1012

#### Description

Makes it easy for everyone to run unit tests, integration tests and all tests as targets using `make`.

Removes the `tests` target, and adds `run-unit-tests`, `run-integration-tests` and `run-all-tests` targets.

Makefile

@@ -19,11 +19,17 @@ client:
 	tox -r --notest -e lint,py3
 	$(PY) -m juju.client.facade -s "juju/client/schemas*" -o juju/client/
 
-.PHONY: test
-test: lint
+.PHONY: run-unit-tests
+run-unit-tests: lint .tox
 	tox -e py3
+
+.PHONY: run-integration-tests
+run-unit-tests: lint .tox
 	tox -e integration
 
+.PHONY: run-all-tests
+test: run-unit-tests run-integration-tests
+
 .PHONY: lint
 lint:
 	@./scripts/copyright.sh