tests: Add a test of relative paths, to make sure we don't break them.

In some places we turn relative paths passed by the caller into
absolute paths: for example in the libvirt backend, all relative paths
to disk images have to be turned into absolute paths (by calling
realpath) because otherwise libvirt doesn't work.

This has the potential to break things, especially considering that
qemu uses the path passed to it in order to resolve the relative paths
of backing files.

For example consider the case where 'overlay' is a symbolic link to
/dev/dm-1, but /dev/dm-1 contains a qcow2 which references './backing'
as its backing file.  If we rewrote 'overlay' to '/dev/dm-1', then
qemu would try to look up '/dev/backing' for the backing file and
fail.

See also:

https://bugzilla.redhat.com/show_bug.cgi?id=1053684
https://www.redhat.com/archives/libguestfs/2014-January/thread.html#00087
Version 1.25.22.