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.
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 4f70799..0093a49 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -2156,7 +2156,7 @@
     def ssl_io_loop(self, sock, incoming, outgoing, func, *args, **kwargs):
         # A simple IO loop. Call func(*args) depending on the error we get
         # (WANT_READ or WANT_WRITE) move data between the socket and the BIOs.
-        timeout = kwargs.get('timeout', 10)
+        timeout = kwargs.get('timeout', support.SHORT_TIMEOUT)
         deadline = time.monotonic() + timeout
         count = 0
         while True: