bpo-38614: Use test.support.SHORT_TIMEOUT constant (GH-17566)

Replace hardcoded timeout constants in tests with SHORT_TIMEOUT of
test.support, so it's easier to ajdust this timeout for all tests at
once.

SHORT_TIMEOUT is 30 seconds by default, but it can be longer
depending on --timeout command line option.

The change makes almost all timeouts longer, except
test_reap_children() of test_support which is made 2x shorter:
SHORT_TIMEOUT should be enough. If this test starts to fail,
LONG_TIMEOUT should be used instead.

Uniformize also "from test import support" import in some test files.

Lib/test/test_wait4.py

@@ -6,11 +6,11 @@ import time
 import sys
 import unittest
 from test.fork_wait import ForkWait
-from test.support import reap_children, get_attribute
+from test import support
 
 # If either of these do not exist, skip this test.
-get_attribute(os, 'fork')
-get_attribute(os, 'wait4')
+support.get_attribute(os, 'fork')
+support.get_attribute(os, 'wait4')
 
 
 class Wait4Test(ForkWait):
@@ -20,7 +20,7 @@ class Wait4Test(ForkWait):
             # Issue #11185: wait4 is broken on AIX and will always return 0
             # with WNOHANG.
             option = 0
-        deadline = time.monotonic() + 10.0
+        deadline = time.monotonic() + support.SHORT_TIMEOUT
         while time.monotonic() <= deadline:
             # wait4() shouldn't hang, but some of the buildbots seem to hang
             # in the forking tests.  This is an attempt to fix the problem.
@@ -33,7 +33,7 @@ class Wait4Test(ForkWait):
         self.assertTrue(rusage)
 
 def tearDownModule():
-    reap_children()
+    support.reap_children()
 
 if __name__ == "__main__":
     unittest.main()