dm: core: Add tests for stringlist functions

These functions currently lack tests so add some. The error handling
differs betwee livetree and flattree at present, so only check the error
codes with livetree.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 8cd688e..e5261bb 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -207,6 +207,7 @@
 		test4-gpios = <&gpio_a 14>, <&gpio_b 4 1 3 2 1>;
 		test5-gpios = <&gpio_a 19>;
 
+		bool-value;
 		int-value = <1234>;
 		uint-value = <(-1234)>;
 		int64-value = /bits/ 64 <0x1111222233334444>;