[INTERNAL] jquery.sap.dom.js: add test case for $.fn.selectText() method

- $.fn.selectText() method should return this to allow method chaining

Change-Id: I8661b923c712d1b16d39ce1eaa5db2cf907a29fa
[INTERNAL] jquery.sap.dom.js: add test case for $.fn.getSelectedText()

- $.fn.getSelectedText() method should return an empty string when the
jQuery collection is empty

Change-Id: I674a4a6bee293f7901dab1f3f20966ac42942c74