…ythonGH-16550) (pythonGH-16560)

* bpo-36670, regrtest: Fix WindowsLoadTracker() for partial line (pythonGH-16550)

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

(cherry picked from commit 3e04cd2)

* bpo-36670: Enhance regrtest WindowsLoadTracker (pythonGH-16553)

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.

(cherry picked from commit c65119d)

* bpo-36670: Enhance regrtest (pythonGH-16556)

* 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'

(cherry picked from commit 098e256)
(cherry picked from commit de3195c)

Co-authored-by: Victor Stinner <vstinner@python.org>