Various fixes from the rust-coreutils/busybox switch

• sed's --unbuffered argument does not exist. Simply dropping it isn't entirely ideal, but there doesn't seem to be any obvious alternative, and I doubt it'll result in any significant regressions.
• For reasons unknown to me, tempfile seems to now be failing due to the test name containing a slash. I'm not sure if this is an area where some differing behavior in ash vs bash resulted in passing a value with a slash, but the easiest workaround is just to replace them with underscores.
  • On that note, I also migrated to mktemp, since tempfile has been deprecated for a long time and was recently removed, plus making the switch was incredibly simple.

https://phabricator.apertis.org/T8423

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com