This utility wasn't very related to all other utilities in pkg/ioutils.
Moving it to longpath to also make it more clear what it does.
It looks like there's only a single (public) external consumer of this
utility, and only used in a test, and it's not 100% clear if it was
intentional to use our package, of if it was a case of "I actually meant
`io/ioutil.MkdirTemp`" so we could consider skipping the alias.
While moving the package, I also renamed `TempDir` to `MkdirTemp`, which
is the signature it matches in "os" from stdlib.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
