Include the timeout value in TimeoutExpired.

This was the original intention, but it wasn't threaded all the way through due
to 'endtime'.  Also added a trivial assertion to get coverage of __str__.
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 3156543..00cf0d5 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -651,7 +651,9 @@
     def test_wait_timeout(self):
         p = subprocess.Popen([sys.executable,
                               "-c", "import time; time.sleep(0.1)"])
-        self.assertRaises(subprocess.TimeoutExpired, p.wait, timeout=0.01)
+        with self.assertRaises(subprocess.TimeoutExpired) as c:
+            p.wait(timeout=0.01)
+        self.assertIn("0.01", str(c.exception))  # For coverage of __str__.
         self.assertEqual(p.wait(timeout=2), 0)