[INTERNAL] sap.m.Dialog: Improve QUnit tests
[INTERNAL] sap.m.Dialog: Improve QUnit tests

- applied "use strict"
- removed unused variables
- removed global variables

Change-Id: I9e42954b765bd72a286ee5467e4ecf37c7799c9f
JIRA: BGSOFUIRODOPI-2545

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5