selftest/devtool: Add modify_localfiles_only test checking symlink path

If recipe uses only file:// fetcher devtool unpacks sources under
oe-local-files/ and adds symlink in source root directory. We need to
verify if the symlink in subdirectory has correct path.

See [YOCTO #13738] for details.

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>