Don't keep exceptions alive after the test finishes

Clearing the exceptions found during a test is needed to allow variables to be properly cleared.

Fixes pytest-dev#187

Author: fabioz

pytestqt/exceptions.py

@@ -58,7 +58,9 @@ def fail_if_exceptions_occurred(self, when):
             exceptions = self.exceptions
             self.exceptions = []
             prefix = '%s ERROR: ' % when
-            pytest.fail(prefix + format_captured_exceptions(exceptions),
+            msg = prefix + format_captured_exceptions(exceptions)
+            del exceptions[:]  # Don't keep exceptions alive longer.
+            pytest.fail(msg,
                         pytrace=False)