expo: Add tests for the configuration editor

Add some simple tests and a helpful script to make the configuration
editor easier to set up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 442222e..b5509ee 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -144,6 +144,12 @@
 	cedit: cedit {
 	};
 
+	cedit-theme {
+		font-size = <30>;
+		menu-inset = <3>;
+		menuitem-gap-y = <1>;
+	};
+
 	fuzzing-engine {
 		compatible = "sandbox,fuzzing-engine";
 	};