Add tests for installroot-relative directories

[evan-goode]

For rpm-software-management/dnf5#1148 and rpm-software-management/dnf5#754

• cachedir and system_cachedir should be relative to (inside) installroot by default
• logdir should be relative to installroot by default
• The configuration file, reposdir, and vars should be taken from inside installroot unless --use-host-config is passed, in which case the configuration and environment from the host system should be used.
• If any of cachedir, system_cachedir, logdir, reposdir, or varsdir are specified on the command line via e.g. --setopt=varsdir=/path/to/varsdir, then the path should be always relative to the host, not the installroot.

See https://dnf5.readthedocs.io/en/latest/misc/installroot.7.html