Include `markSelected` in `getSelections` test
Include `markSelected` in `getSelections` test

`markSelected` was previously stubbed but the
side effect of changing the `selections` property
changes how it runs so the actual method needs to
be included to ensure this change is correct.

TFOH/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit