test: emergency patch to repair the Windows CI

The removal of a key not in the set is an error.

Author: compnerd

test/lit.cfg

@@ -1500,7 +1500,8 @@ else:
         config.available_features.add('libdispatch')
     else:
         # TSan runtime requires libdispatch on non-Apple platforms
-        config.available_features.remove('tsan_runtime')
+        if 'tsan_runtime' in config.available_features:
+            config.available_features.remove('tsan_runtime')
 
     if has_lib('Foundation'):
         config.available_features.add('foundation')