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/signalinterproctester.py

@@ -4,6 +4,7 @@ import subprocess
 import sys
 import time
 import unittest
+from test import support
 
 
 class SIGUSR1Exception(Exception):
@@ -27,7 +28,7 @@ class InterProcessSignalTests(unittest.TestCase):
             # (if set)
             child.wait()
 
-        timeout = 10.0
+        timeout = support.SHORT_TIMEOUT
         deadline = time.monotonic() + timeout
 
         while time.monotonic() < deadline: