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_socketserver.py

@@ -36,7 +36,7 @@ def signal_alarm(n):
 # Remember real select() to avoid interferences with mocking
 _real_select = select.select
 
-def receive(sock, n, timeout=20):
+def receive(sock, n, timeout=test.support.SHORT_TIMEOUT):
     r, w, x = _real_select([sock], [], [], timeout)
     if sock in r:
         return sock.recv(n)