The logic for `--selinux-policy` ended up in the `--tree=dir`
path, but there's no reason for that. Fix the imported
labeling with `--tree=tar`. Prep for use with containers.
We had this bug because the previous logic was trying to avoid
duplicating the code for generic `--selinux-policy` and
the case of `--selinux-policy-from-base --tree=dir`.
It's a bit more code, but it's cleaner if we dis-entangle them.
Follow the precedent set in https://github.com/coreos/rpm-ostree/pull/2106
and rename the directory, to more clearly move away from the
"uninstalled" test model. Prep for Rust-based tests.
Colin Walters