[iOS] Disable testNTPStayForInvalidURL for iOS 12

The test is flaky on iOS 12.

Fixed: 1095534
Change-Id: I31acfa4187a3e14edd76914131d08b35beff30ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2396115
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804870}

ios/chrome/browser/ui/ntp/new_tab_page_egtest.mm

@@ -53,6 +53,10 @@ BOOL WaitForHistoryToDisappear() {
 
 // Tests that the NTP is still displayed after loading an invalid URL.
 - (void)testNTPStayForInvalidURL {
+  if (@available(iOS 13, *)) {
+  } else {
+    EARL_GREY_TEST_DISABLED(@"Failing on iOS 12.");
+  }
 // TODO(crbug.com/1067813): Test won't pass on iPad device.
 #if !TARGET_IPHONE_SIMULATOR
   if ([ChromeEarlGrey isIPadIdiom]) {