tests/helper-runtests.sh

context: use `dirstate.set_tracked` in context.copy

This is the new shiny API.

Differential Revision: https://phab.mercurial-scm.org/D11015

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL