Amend test coverage of tab selection editor

When developing https://crrev.com/c/2118621, we found that if the
following line is deleted, no tests would fail.

https://source.chromium.org/chromium/chromium/src/+/master:chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/TabSwitcherCoordinator.java;l=189;drc=556bf45e2c50857b400b30ed4b74ed6617091126

This CL amends the test coverage so that if that line is missing,
testTabSuggestionMessageCard_review() would fail.

Bug: 1041865
Change-Id: I774b796bf97b0a40789d365d4d0a7ebee055afff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2121763Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753671}

chrome/android/features/start_surface/internal/javatests/src/org/chromium/chrome/features/start_surface/StartSurfaceLayoutTest.java

@@ -830,6 +830,9 @@ public class StartSurfaceLayoutTest {
         TabSelectionEditorTestingRobot tabSelectionEditorTestingRobot =
                 new TabSelectionEditorTestingRobot();
         tabSelectionEditorTestingRobot.resultRobot.verifyTabSelectionEditorIsVisible();
+
+        Espresso.pressBack();
+        tabSelectionEditorTestingRobot.resultRobot.verifyTabSelectionEditorIsHidden();
     }
 
     @Test