regrtest: mention in tests run sequentially or in parallel

Lib/test/libregrtest/main.py

@@ -305,6 +305,8 @@ class Regrtest:
 
         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()

Lib/test/libregrtest/runtest_mp.py

@@ -154,6 +154,8 @@ def run_tests_multiprocess(regrtest):
 
     workers = [MultiprocessThread(pending, output, regrtest.ns)
                for i in range(regrtest.ns.use_mp)]
+    print("Run tests in parallel using %s child processes"
+          % len(workers))
     for worker in workers:
         worker.start()