tests: don't save host fingerprints in hgrc
Previously, the test saved the host fingerprints in hgrc. Many tests
override the fingerprint at run-time. This was a bit dangerous and
was too magical for my liking. It will also interfere with a future
patch that adds a new source for obtaining fingerprints.
So change the test to require the fingerprint on every command
invocation.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL