Bug fix on error suppression and comment

src/objective-c/tests/Podfile

@@ -82,7 +82,11 @@ post_install do |installer|
     target.build_configurations.each do |config|
       config.build_settings['GCC_TREAT_WARNINGS_AS_ERRORS'] = 'YES'
     end
-    if target.name.start_with?('gRPC-Core')
+
+    # CocoaPods creates duplicated library targets of gRPC-Core when the test targets include
+    # non-default subspecs of gRPC-Core. All of these library targets start with prefix 'gRPC-Core.'
+    # and require the same error suppresion.
+    if target.name == 'gRPC-Core' or target.name.start_with?('gRPC-Core.') 
       target.build_configurations.each do |config|
         # TODO(zyc): Remove this setting after the issue is resolved
         # GPR_UNREACHABLE_CODE causes "Control may reach end of non-void