testsuite/path: defer and inline get_rootpath()

testsuite/path: defer and inline get_rootpath()

In a handful of use-cases, we don't need to trap the respective libc
function. Although at that point we have already fetched the rootfs
environment variable.

Defer it until it's needed and in the process inline the function. As
result we have a) less duplication in the wrappers and b) the rootpath
handling is no longer split across multiple functions.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/355
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>