tests/helper-runtests.sh

fastannotate: slice strings to get single character

Behaves identically on Python 3 and Python 2.

Differential Revision: https://phab.mercurial-scm.org/D5612

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL