dm: core: Add tests for dev_read_alias_highest_id()

It is checking the highest alias ID for eth, gpio, pci, i2c and error
code on non existing alias.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/test-fdt.c b/test/dm/test-fdt.c
index 984b80c..be16c99 100644
--- a/test/dm/test-fdt.c
+++ b/test/dm/test-fdt.c
@@ -219,6 +219,29 @@
 }
 DM_TEST(dm_test_fdt, 0);
 
+static int dm_test_alias_highest_id(struct unit_test_state *uts)
+{
+	int ret;
+
+	ret = dev_read_alias_highest_id("eth");
+	ut_asserteq(5, ret);
+
+	ret = dev_read_alias_highest_id("gpio");
+	ut_asserteq(2, ret);
+
+	ret = dev_read_alias_highest_id("pci");
+	ut_asserteq(2, ret);
+
+	ret = dev_read_alias_highest_id("i2c");
+	ut_asserteq(0, ret);
+
+	ret = dev_read_alias_highest_id("deadbeef");
+	ut_asserteq(-1, ret);
+
+	return 0;
+}
+DM_TEST(dm_test_alias_highest_id, 0);
+
 static int dm_test_fdt_pre_reloc(struct unit_test_state *uts)
 {
 	struct uclass *uc;