disable flaky showcase tests

Disabling following tests because they are flaky on devices (failed 5
times during the past 20 runs):

SCTabTestCase/testLaunchWithTopToolbar
SCTabTestCase/testLaunchWithBottomToolbar
SCTabTestCase/testLaunchAndTappingCell

TBR=lpromero@chromium.org
BUG=710662

Review-Url: https://codereview.chromium.org/2815773002
Cr-Commit-Position: refs/heads/master@{#463831}

ios/showcase/tab/sc_tab_egtest.mm

@@ -18,13 +18,15 @@
 @implementation SCTabTestCase
 
 // Tests launching TopToolbarTabViewController.
-- (void)testLaunchWithTopToolbar {
+// TODO(crbug.com/710662): re-enable this test.
+- (void)FLAKY_testLaunchWithTopToolbar {
   showcase_utils::Open(@"TopToolbarTabViewController");
   showcase_utils::Close();
 }
 
 // Tests launching BottomToolbarTabViewController.
-- (void)testLaunchWithBottomToolbar {
+// TODO(crbug.com/710662): re-enable this test.
+- (void)FLAKY_testLaunchWithBottomToolbar {
   showcase_utils::Open(@"BottomToolbarTabViewController");
   showcase_utils::Close();
 }

ios/showcase/tab_grid/sc_tab_grid_egtest.mm

@@ -18,7 +18,8 @@
 @implementation SCTabGridTestCase
 
 // Tests launching TabGridViewController and tapping a cell.
-- (void)testLaunchAndTappingCell {
+// TODO(crbug.com/710662): re-enable this test.
+- (void)FLAKY_testLaunchAndTappingCell {
   showcase_utils::Open(@"TabGridViewController");
   [[EarlGrey selectElementWithMatcher:grey_accessibilityID(@"Tab 0_button")]
       performAction:grey_tap()];