efi_selftest: LocateHandleBuffer return code

Check that LocateHandleBuffer() return EFI_NOT_FOUND when called with
ByRegisterNotify and all handles already have been retrieved.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>