Allow radio selection buttons that are checked

Selection buttons did not support radio buttons that were checked on page-load because of an error with the order of code in the `getSelections` method. 

The previous order meant the `markSelected` method was called on the checked radio button before we stored a reference to its group. This meant the method failed silently so the radio button was not being marked as checked.

This corrects the order and includes the `markSelected` method in the relevant test to ensure it works as expected.