tests/helper-runtests.sh

internal-doc: document the config register mechanism

This explains the various usage and feature of the config register introduced
in Mercurial 4.3 and 4.4.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL