maint: Refactor suntab and aixtab filetype

Apply the create/do_stuff/delete cycle of a temp file that is explained
in tempfile.rb to the suntab and aixtab filetype. This way the temporary
file should always be removed.

The name of the temporary file was changed so it will be easier to stub
this single call in tests - without stubbing tempfile creations in other
places (like inside Puppet::Util.execute) at the same time.