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_signal.py b/Lib/test/test_signal.py
index 119e9e0..5c02c28 100644
--- a/Lib/test/test_signal.py
+++ b/Lib/test/test_signal.py
@@ -644,7 +644,7 @@
                 # wait until the child process is loaded and has started
                 first_line = process.stdout.readline()
 
-                stdout, stderr = process.communicate(timeout=5.0)
+                stdout, stderr = process.communicate(timeout=support.SHORT_TIMEOUT)
             except subprocess.TimeoutExpired:
                 process.kill()
                 return False
@@ -1192,7 +1192,7 @@
         self.setsig(signal.SIGALRM, second_handler)  # for ITIMER_REAL
 
         expected_sigs = 0
-        deadline = time.monotonic() + 15.0
+        deadline = time.monotonic() + support.SHORT_TIMEOUT
 
         while expected_sigs < N:
             os.kill(os.getpid(), signal.SIGPROF)
@@ -1226,7 +1226,7 @@
         self.setsig(signal.SIGALRM, handler)  # for ITIMER_REAL
 
         expected_sigs = 0
-        deadline = time.monotonic() + 15.0
+        deadline = time.monotonic() + support.SHORT_TIMEOUT
 
         while expected_sigs < N:
             # Hopefully the SIGALRM will be received somewhere during