efi_selftest: correctly cleanup after selftest

After executing bootefi selftest
* restore GD
* unlink the load image handle
* return 0 or 1 and not a truncated efi_status_t.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>