tests/helper-runtests.sh

hgweb: use computed base URL from parsed request

Let's not reinvent URL construction in a function that runs the
templater.

Differential Revision: https://phab.mercurial-scm.org/D2735

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL