tests: don't create a syslinux stub unconditionally in libtest

tests: don't create a syslinux stub unconditionally in libtest

libtest.sh has an setup_os_repository() helper function tha is
used by many tests to setup an OSTree initial repository.

This function creates an syslinux configuration unconditionally
but OSTree supports other bootloader backends besides syslinux.

So, is better to conditionally create a syslinux configuration
only when it is needed.

https://bugzilla.gnome.org/show_bug.cgi?id=708069

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>