[ios] Reworks +setUpForTestCase to be called once per XCTestCase.

The previous implementation was inadvertently calling +setUpForTestCase
once per test module, rather than once per XCTestCase.

BUG=922813,977161

Change-Id: I49d0f5bc442c4bd793b85f932149699d24f5e665
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692527Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675999}

ios/testing/earl_grey/base_earl_grey_test_case.mm

@@ -22,6 +22,15 @@
 GREY_STUB_CLASS_IN_APP_MAIN_QUEUE(BaseEarlGreyTestCaseAppInterface)
 #endif  // defined(CHROME_EARL_GREY_2)
 
+namespace {
+
+// If true, +setUpForTestCase will be called from -setUp.  This flag is used to
+// ensure that +setUpForTestCase is called exactly once per unique XCTestCase
+// and is reset in +tearDown.
+bool g_needs_set_up_for_test_case = true;
+
+}  // namespace
+
 @implementation BaseEarlGreyTestCase
 
 + (void)setUpForTestCase {
@@ -46,11 +55,16 @@ GREY_STUB_CLASS_IN_APP_MAIN_QUEUE(BaseEarlGreyTestCaseAppInterface)
   [self failIfSetUpIsOverridden];
 #endif
 
-  static dispatch_once_t setupToken;
-  dispatch_once(&setupToken, ^{
+  if (g_needs_set_up_for_test_case) {
+    g_needs_set_up_for_test_case = false;
     [CoverageUtils configureCoverageReportPath];
     [[self class] setUpForTestCase];
-  });
+  }
+}
+
++ (void)tearDown {
+  g_needs_set_up_for_test_case = true;
+  [super tearDown];
 }
 
 // Handles system alerts if any are present, closing them to unblock the UI.