t/hl_mod.t: remove IPC::Run (and File::Temp) dependency

t/hl_mod.t: remove IPC::Run (and File::Temp) dependency

We already load PublicInbox::Spawn for which(), so using spawn()
isn't unreasonable.  And rely on "skip" to log the omitted test
if w3m is missing, which means we need to update the "&&"
escaping test to be self-referential on the same line.

File::Temp was totally unused, there; and we can use "open ...,undef"
in Perl to easily create anonymous temporary files for use with
spawn().