Create temporary directory under /tmp/uhyve instead of /tmp/

[n0toose]

• This will allow removing randomly generated directories in their entirety (their file contents are removed) after they are discarded, see: Introduce Landlock isolation support #816 (comment)
• By giving R/W access to /tmp/uhyve/ (but explicitly retaining the 0o700 permission), it will be possible to create new temporary directories (with randomly generated names), which is useful for the multicore test and Landlock: Introduce Landlock isolation support #816 (review)
  • The multicore test starts multiple instances of Uhyve sequentially, thus attempts to generate more than one temporary directory and fails.
  • Another potential workaround is creating a series of tests that run sequentially, or using a macro to automatically generate them without adding lots of duplicate code.
  • Another potential workaround is exploring forking / spawning a new process inside of the test itself.

[n0toose]

Alternative (perhaps much more viable) proposal: Do not create a new temporary directory if one has already been created. Remove the temporary directory's contents before "recycling it". That way, Landlock won't freak out (and everything remains stateless enough for the purposes of testing).