regrtest: mention in tests run sequentially or in parallel
diff --git a/Lib/test/libregrtest/main.py b/Lib/test/libregrtest/main.py
index de08f32..c6d9ad0 100644
--- a/Lib/test/libregrtest/main.py
+++ b/Lib/test/libregrtest/main.py
@@ -305,6 +305,8 @@
 
         save_modules = sys.modules.keys()
 
+        print("Run tests sequentially")
+
         previous_test = None
         for test_index, test in enumerate(self.tests, 1):
             start_time = time.monotonic()