MDL-72172 reportbuilder: fix cohort report source tests.

Choose aggregation method that has availability across all
supported databases; and allows sorting.

"Comma separated distinct values" doesn't fulfill either of
these criteria.

cohort/tests/behat/reportbuilder.feature

@@ -83,18 +83,18 @@ Feature: Manage custom reports for cohorts
     Given the following "core_reportbuilder > Columns" exist:
       | report    | uniqueidentifier   |
       | My report | user:lastname      |
+    And I change window size to "large"
     And I am on the "My report" "reportbuilder > Editor" page logged in as "admin"
-    And I set the field "Rename column 'Surname'" to "Members"
-    And I reload the page
-    And I set the field "Aggregate column 'Surname'" to "Comma separated distinct values"
+    And I set the field "Rename column 'Surname'" to "Member count"
+    And I set the "Surname" column aggregation to "Count"
     And I click on "Show/hide settings sidebar" "button"
     And I click on "Show/hide 'Sorting'" "button"
-    And I change window size to "large"
     And I click on "Move sorting for column 'Surname'" "button"
     And I click on "To the top of the list" "link" in the "Move sorting for column 'Surname'" "dialogue"
     And I click on "Enable sorting for column 'Surname'" "checkbox"
-    And "Another one" "table_row" should appear before "MDL-62161" "table_row"
+    # "New system cohort" has fewer members than "Another one" cohort.
+    And "New system cohort" "table_row" should appear before "Another one" "table_row"
     When I click on "Sort column 'Surname' descending" "button"
     Then I should see "Updated sorting for column 'Surname'"
-    And I wait "1" seconds
-    And "MDL-62161" "table_row" should appear before "Another one" "table_row"
+    # Switching sort direction should now show "Another one" cohort ahead of "New system cohort".
+    And "Another one" "table_row" should appear before "New system cohort" "table_row"