Fail hard when tests pass that are expected to fail

`@pytest.mark.xfail()` can be used to decorate tests that don't yet
pass. By default, if I wrote a test like this:

```
@pytest.mark.xfail()
def test_naughty():
    assert 1 == 1
```

Then Pytest would allow this test to pass without notifying that an
expected-fail *didn't* fail. To make that happen, you have to set
xfail_strict=True so that the above test would cause the tests to fail.

tox.ini

@@ -20,6 +20,8 @@ passenv =
     # variable error. See issue: #20424
     HOME
 
+[pytest]
+xfail_strict = true
 
 [flake8]
 # These are things that the devs don't agree make the code more readable