Improve tests for methodFromReferrerScope

From gerrit: https://codereview.qt-project.org/c/qt/qtdeclarative/+/513549/comment/225c66f3_5044bd35/

One test in tst_qmlls_utils::findDefinitionFromLocation that asks for the definition of a signal handler, another for the definition of a property changed handler.
Also another two tests in tst_qmlls_utils::resolveExpressionType for the type of a signal handler and the type of a property changed handler.