tests/helper-runtests.sh

templatefilters: drop callable support from json()

This backs out ae5447de4c11. A callable should be evaluated beforehand
by templater.runsymbol().

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL