lfs: generate a large file by using `python` instead of yes | head
yes(1) on some systems (like gcc112) feels compelled to inform you of
broken pipes, such as those triggered by head(1). This works around
the problem portably.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL