fix off-by-one error in _alpm_filelist_resolve

'/' should not be appended to the resolved root when root is "/".

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
pmtest: install filesystem entries before packages

Installing filesystem entries first allows the filesystem to provide
a symlink to a directory.  Packages will then be able to use the symlink
as if it were a directory instead of causing an error.

For example:

self.filesystem = ["dir/", "link -> dir/"]
pkg = pmpkg("pkg1")
pkg.files = ["link/file"]
self.addpkg2db("local", pkg)

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>