Add more thorough tests for the template search

It would be quite easy to dissociate the search box from the things its
supposed to be searching.

This commit adds assertions to make sure that the things the search box
is targeting are on the page

tests/app/main/views/test_templates.py

@@ -173,8 +173,12 @@ def test_should_show_live_search_if_list_of_templates_taller_than_screen(
         'main.choose_template',
         service_id=SERVICE_ONE_ID,
     )
+    search = page.select_one('.live-search')
 
-    assert page.select('.live-search')
+    assert search['data-module'] == 'live-search'
+    assert search['data-targets'] == '#template-list .column-whole'
+
+    assert len(page.select(search['data-targets'])) == len(page.select('.message-name')) == 14
 
 
 def test_should_show_page_for_one_template(