tests/helper-runtests.sh

byteify-strings: handle multi-line strings in _ensuresysstr

The current implementation did not handle calls like

`repo.ui.log("first line"
             "other line")`

correctly.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL