Fail tests on Python warnings.

Fixes #982.

tests/conftest.py

@@ -27,6 +27,7 @@ import collections
 import itertools
 import logging
 import textwrap
+import warnings
 
 import pytest
 
@@ -309,3 +310,10 @@ def py_proc():
     def func(code):
         return (sys.executable, ['-c', textwrap.dedent(code.strip('\n'))])
     return func
+
+
+@pytest.yield_fixture(autouse=True)
+def fail_tests_on_warnings():
+    warnings.simplefilter('error')
+    yield
+    warnings.resetwarnings()

tests/test_conftest.py

@@ -20,6 +20,11 @@
 """Various meta-tests for conftest.py."""
 
 
+import warnings
+
+import pytest
+
+
 def test_qapp_name(qapp):
     """Make sure the QApplication name is changed when we use qapp."""
     assert qapp.applicationName() == 'qute_test'
@@ -28,3 +33,8 @@ def test_qapp_name(qapp):
 def test_no_qapp(request):
     """Make sure a test without qapp doesn't use qapp (via autouse)."""
     assert 'qapp' not in request.fixturenames
+
+
+def test_fail_on_warnings():
+    with pytest.raises(PendingDeprecationWarning):
+        warnings.warn('test', PendingDeprecationWarning)