adding -a option to xvfb-run, and putting xvfb-run in

envlist - which is like a whitelist for commands, I think.

Makefile

@@ -61,7 +61,7 @@ test-all: ## run tests on every Python version with tox
 	tox
 
 coverage: ## check code coverage quickly with the default Python
-	xvfb-run coverage run --source flatland -m pytest
+	xvfb-run -a coverage run --source flatland -m pytest
 	coverage report -m
 	coverage html
 	$(BROWSER) htmlcov/index.html

tox.ini

 [tox]
-envlist = py36, py37, flake8, docs, coverage
+envlist = py36, py37, flake8, docs, coverage, xvfb-run, sh
 
 [travis]
 python =
@@ -36,6 +36,6 @@ deps =
 commands =
     pip install -U pip
     sh -c 'echo DISPLAY: $DISPLAY'
-    xvfb-run py.test --basetemp={envtmpdir}
+    xvfb-run -a py.test --basetemp={envtmpdir}