ui_test: simplify assertion

This step just checks that multi relation field listbox shows a list of
related document and that clicking on any item of the list leads to the
related document.
The order of related documents in the list has always been unspecified,
but the first one was always 1 until recently.
Because this test does not really care which one of the two related
document was clicked, just assert that it's a document from foo_module.