tests: Introduce DT overlay tests

This adds a bunch of unit tests for the "fdt apply" command.

They've all been run successfully in the sandbox. However, as you still
require an out-of-tree dtc with overlay support, this is disabled by
default.

Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/test/Kconfig b/test/Kconfig
index d71c332..3643761 100644
--- a/test/Kconfig
+++ b/test/Kconfig
@@ -17,3 +17,4 @@
 
 source "test/dm/Kconfig"
 source "test/env/Kconfig"
+source "test/overlay/Kconfig"