bpo-30258: regrtest handles child process crash (#1431)

bpo-30258: regrtest handles child process crash (#1431)

Backport the CHILD_ERROR status from master: a test is considered as
failed if a worker process running a test exited with a code
different than zero.

Change also the output: write stdout and stderr of the child process
after the test name, instead of writing it before.

accumulate_result(): don't use time of CHILD_ERROR or INTERRUPTED
results.