dm: core: Drop header files from dm/test.h

These header file should not be included in other header files. Remove
them and add to each individual file. Add test/test.h to test/ui.h since
that is a reasonable place.

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

test/dm/serial.c

@@ -8,6 +8,7 @@
 #include <serial.h>
 #include <dm.h>
 #include <dm/test.h>
+#include <test/test.h>
 #include <test/ut.h>
 
 static int dm_test_serial(struct unit_test_state *uts)