New test function replace_conf() to get rid of unlink/append pattern

We have a fairly common pattern in the TAP tests where we first unlink() a
config file and then do append_conf() on it in order to rewrite it from
scratch. There's nothing wrong with it but it's IMO not as readable as it can
be (especially when unlink and append_conf are vertically separated), and
slightly errorprone as the unlink requires one to assemble the path whereas
append_conf does that automatically (yes, I have done that exact mistake during
hacking).

To make the TAP tests more readable I propose a utility function which combines
these two operations, replace_conf(). It works just like how unlink() together
with append_conf() does but in fewer and more readable lines of code. I chose
replace rather than rewrite since it IMO more clearly signals that nothing in
the file is kept.

--
Daniel Gustafsson