test: dm: restore /firmware nodes after testing

dm_test_restore() is called after dm unit test is run.
But this function does not scan any nodes under /firmware since
it calls dm_scan_fdt().

This causes an issue. For instance, scmi_sandbox_agent device
will disappear after running 'ut dm scmi_sandbox_agent'.

So call dm_extended_scan() instead. This change will be coherent
with what dm_scan() and test_pre_run() does.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>