tests/helper-runtests.sh

tests: fix test-casefolding.t

The message had changed, but the test was not updated. This test does not
run on Linux, but failed on my Mac.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL