tests: Empty clipboard after testing if it works.

Otherwise, if a test fails to actually put something into the clipboard, we end
up pasting "Does this work?" which could e.g. trigger a search.

When it's cleared, we at least get some "clipboard is empty" error instead.

tests/integration/features/test_yankpaste.py

@@ -39,6 +39,9 @@ def skip_with_broken_clipboard(qtbot, qapp):
     if clipboard.text() != "Does this work?":
         pytest.skip("Clipboard seems to be broken on this platform.")
 
+    with qtbot.waitSignal(clipboard.changed):
+        clipboard.clear()
+
 
 @bdd.when(bdd.parsers.parse('I set the text field to "{value}"'))
 def set_text_field(quteproc, value):