Log "Warning -- ..." test warnings into sys.__stderr__ rather than
sys.stderr, to ensure to display them even if sys.stderr is captured.

test.libregrtest.utils.print_warning() now calls
test.support.print_warning().

Reduce also worker timeout.

When building Python in some uncommon platforms there are some known tests that will fail. Right now, the test suite has the ability to ignore entire tests using the -x option and to receive a filter file using the --matchfile filter. The problem with the --matchfile option is that it receives a file with patterns to accept and when you want to ignore a couple of tests and subtests, is too cumbersome to lists ALL tests that are not the ones that you want to accept and he problem with -x is that is not easy to ignore just a subtests that fail and the whole test needs to be ignored.

For these reasons, add a new option to allow to ignore a list of test and subtests for these situations.

Add timeout constants to test.support:

* LOOPBACK_TIMEOUT
* INTERNET_TIMEOUT
* SHORT_TIMEOUT
* LONG_TIMEOUT

test.regrtest now uses process groups in the multiprocessing mode
(-jN command line option) if process groups are available: if
os.setsid() and os.killpg() functions are available.

Fix a warning on a race condition on TestWorkerProcess.kill(): ignore
silently ProcessLookupError rather than logging an useless warning.

bpo-37531, bpo-38207: On timeout, regrtest no longer attempts to call
`popen.communicate() again: it can hang until all child processes
using stdout and stderr pipes completes. Kill the worker process and
ignores its output.

Reenable test_regrtest.test_multiprocessing_timeout().

bpo-37531: Change also the faulthandler timeout of the main process
from 1 minute to 5 minutes, for Python slowest buildbots.

* Add log() method: add timestamp and load average prefixes
  to main messages.
* WindowsLoadTracker:

  * LOAD_FACTOR_1 is now computed using SAMPLING_INTERVAL
  * Initialize the load to the arithmetic mean of the first 5 values
    of the Processor Queue Length value (so over 5 seconds), rather
    than 0.0.
  * Handle BrokenPipeError and when typeperf exit.

* format_duration(1.5) now returns '1.5 sec', rather than
  '1 sec 500 ms'

The last line is now passed to the parser even if it does not end
with a newline, but only if it's a valid value.

WindowsLoadTracker.read_output() now uses a short buffer for
incomplete line.