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>
diff --git a/test/dm/timer.c b/test/dm/timer.c
index 9367dab..4aa5eea 100644
--- a/test/dm/timer.c
+++ b/test/dm/timer.c
@@ -7,6 +7,7 @@
 #include <dm.h>
 #include <timer.h>
 #include <dm/test.h>
+#include <test/test.h>
 #include <test/ut.h>
 
 /*