Turn introspection on in the main_window fixture

It is not clear why this is needed, but without this the tests
abort on exit with "QThread: Destroyed while thread is still running"

spyder_unittest/tests/conftest.py

@@ -38,6 +38,12 @@ def main_window(monkeypatch):
 
     # Don't show tours message
     CONF.set('tours', 'show_tour_message', False)
+
+    # Turn introspection on, even though it's slower and more memory
+    # intensive, because otherwise tests are aborted at end with
+    # "QThread: Destroyed while thread is still running".
+    os.environ['SPY_TEST_USE_INTROSPECTION'] = 'True'
+
     QApplication.processEvents()
 
     # Start the window
@@ -52,3 +58,5 @@ def main_window(monkeypatch):
     CONF.reset_to_defaults(notification=False)
     CONF.reset_manager()
     PLUGIN_REGISTRY.reset()
+
+    os.environ.pop('SPY_TEST_USE_INTROSPECTION')