Add test cases for rocketchataccountsettings

Patch from Veluri Mithun. Thanks

Summary:
Add defaultValues test case

Add Assign/get test cases for all the methods

Test Plan: Use QsignalSpy to test signals

Reviewers: Ruqola, mlaurent

Reviewed By: mlaurent

Differential Revision: https://phabricator.kde.org/D9390