[ci] Enforce Swift strict concurrency during testing

Author: jmagman

script/tool/lib/src/native_test_command.dart

@@ -564,8 +564,10 @@ this command.
         extraFlags: <String>[
           if (testTarget != null) '-only-testing:$testTarget',
           ...extraFlags,
-          if (!_xcodeWarningsExceptions.contains(plugin.directory.basename))
+          if (!_xcodeWarningsExceptions.contains(plugin.directory.basename)) ...<String>[
             'GCC_TREAT_WARNINGS_AS_ERRORS=YES',
+            r'OTHER_SWIFT_FLAGS=$(inherited) -strict-concurrency=complete',
+          ],
         ],
       );
 

script/tool/test/native_test_command_test.dart

@@ -177,7 +177,10 @@ void main() {
         'Debug',
         if (destination != null) ...<String>['-destination', destination],
         ...extraFlags,
-        if (treatWarningsAsErrors) 'GCC_TREAT_WARNINGS_AS_ERRORS=YES',
+        if (treatWarningsAsErrors) ...<String>[
+          'GCC_TREAT_WARNINGS_AS_ERRORS=YES',
+          r'OTHER_SWIFT_FLAGS=$(inherited) -strict-concurrency=complete',
+        ],
       ], package.path);
     }