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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true