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_wait3.py

@@ -18,7 +18,8 @@ class Wait3Test(ForkWait):
         # This many iterations can be required, since some previously run
         # tests (e.g. test_ctypes) could have spawned a lot of children
         # very quickly.
-        for i in range(30):
+        deadline = time.monotonic() + 10.0
+        while time.monotonic() <= deadline:
             # wait3() 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, rusage = os.wait3(os.WNOHANG)