pytest: add sandbox test for "extension" command

This commit extends the sandbox to implement a dummy
extension_board_scan() function and enables the extension command in
the sandbox configuration. It then adds a test that checks the proper
functionality of the extension command by applying two Device Tree
overlays to the sandbox Device Tree.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
[trini: Limit to running on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/sandbox/dts/overlay0.dts b/arch/sandbox/dts/overlay0.dts
new file mode 100644
index 0000000..70c6cf7
--- /dev/null
+++ b/arch/sandbox/dts/overlay0.dts
@@ -0,0 +1,9 @@
+/dts-v1/;
+/plugin/;
+
+&{/buttons} {
+	btn3 {
+		gpios = <&gpio_a 5 0>;
+		label = "button3";
+	};
+};