sandbox: Add a way to reset sandbox state for tests

Running a new test should reset the sandbox state to avoid tests
interferring with each other. Move the existing state-reset code into a
function so it can be used from tests.

Also update the code to reset the SPI devices and adjust the test code to
call it.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/test-main.c

@@ -29,6 +29,7 @@ static int dm_test_init(struct unit_test_state *uts)
 	memset(dms, '\0', sizeof(*dms));
 	gd->dm_root = NULL;
 	memset(dm_testdrv_op_count, '\0', sizeof(dm_testdrv_op_count));
+	state_reset_for_test(state_get_current());
 
 	ut_assertok(dm_init(false));
 	dms->root = dm_root();