[3.8] bpo-36670, regrtest: Fix WindowsLoadTracker() for partial line (GH-16550) ...

[3.8] bpo-36670, regrtest: Fix WindowsLoadTracker() for partial line (GH-16550) (GH-16560)

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

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

(cherry picked from commit 3e04cd268ee9a57f95dc78d8974b21a6fac3f666)

* bpo-36670: Enhance regrtest WindowsLoadTracker (GH-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 c65119d5bfded03f80a9805889391b66fa7bf551)

* bpo-36670: Enhance regrtest (GH-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 098e25672f1c3578855d5ded4f5147795c9ed956)
(cherry picked from commit de3195c937d5fca0d79cc93dbafa76c0f89ca5b8)

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