Issue #20910: Make tests more reliable, less dependent on time

* Tolerate 10 seconds instead of 3 seconds for slow test
* Faster test, use sleep of 100 ms instead of 1 sec
* Replace a number of iterations with an explicit deadline for the timeout

Lib/test/test_fork1.py

@@ -18,13 +18,14 @@ get_attribute(os, 'fork')
 
 class ForkTest(ForkWait):
     def wait_impl(self, cpid):
-        for i in range(10):
+        deadline = time.monotonic() + 10.0
+        while time.monotonic() <= deadline:
             # waitpid() shouldn't hang, but some of the buildbots seem to hang
             # in the forking tests.  This is an attempt to fix the problem.
             spid, status = os.waitpid(cpid, os.WNOHANG)
             if spid == cpid:
                 break
-            time.sleep(1.0)
+            time.sleep(0.1)
 
         self.assertEqual(spid, cpid)
         self.assertEqual(status, 0, "cause = %d, exit = %d" % (status&0xff, status>>8))