[3.14] gh-74112: Fix race in test_interrupt_blocking_call (GH-158914) (GH-158920)

On a loaded machine the KeyboardInterrupt could be raised before the main
thread reached time.sleep(), escape the test and stop the whole regrtest run.
(cherry picked from commit ea0ee92)

Co-authored-by: Christian Aurich Zanettini Martins <christian.aurichzm@gmail.com>

Author: miss-islington

Lib/idlelib/idle_test/test_run.py

@@ -546,9 +546,11 @@ def test_interrupt_blocking_call(self):
         # gh-74112: interrupt the main thread blocked in time.sleep().
         timer = threading.Timer(0.1, self.ex.interrupt_the_server)
         self.addCleanup(timer.join)
-        timer.start()
         start = time.monotonic()
         with self.assertRaises(KeyboardInterrupt):
+            # On a loaded machine the signal can arrive before the main
+            # thread reaches time.sleep(), so start the timer in the block.
+            timer.start()
             time.sleep(support.SHORT_TIMEOUT)
         self.assertLess(time.monotonic() - start, support.SHORT_TIMEOUT / 2)