pytest.ini: Don't print captured output

It makes the summary of passes and fails more annoying to read due
to the lines being long and including newlines. I am actually not
sure why this fixes it, though...

See: https://pytest.readthedocs.io/en/latest/capture.html

pytest.ini

@@ -1,4 +1,4 @@
 [pytest]
-addopts= -rsxX -s -v --strict
+addopts= -rsxX -s -v --strict --capture=sys
 filterwarnings =
     error::UserWarning