test: Add tests for the extcon

Provide tests to the simple extcon device.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/extcon/Makefile b/drivers/extcon/Makefile
index 2510e91..c4223d9 100644
--- a/drivers/extcon/Makefile
+++ b/drivers/extcon/Makefile
@@ -3,3 +3,4 @@
 # Copyright (C) 2023 Svyatoslav Ryhel <clamor95@gmail.com>
 
 obj-$(CONFIG_EXTCON) += extcon-uclass.o
+obj-$(CONFIG_EXTCON_SANDBOX) += extcon-sandbox.o