tests: remove unnecessary explicit args from `failfilemerge.py`
The `failfilemerge()` function always raises an exception without
looking at its arguments, so let's use `*args` and `**kwargs`.
Differential Revision: https://phab.mercurial-scm.org/D11854
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[experimental]
changegroup3 = True
EOF