tests/helper-runtests.sh

pycompat: remove json.loads polyfill for Python 3.5

We no longer support Python 3.5 so this can be deleted.

Differential Revision: https://phab.mercurial-scm.org/D12364

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL