tests/helper-runtests.sh

lfs: add the ability to disable the usercache

While the usercache is important for real world uses, I've been tripped up more
than a couple of times by it in tests- thinking a file was being downloaded, but
it was simply linked from the local cache.  The syntax for setting it is the
same as for setting a null remote endpoint, and like that endpoint, is left
undocumented.

This may or may not be a useful feature in the real world (I'd expect any sane
filesystem to support hardlinks at this point).

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL