test: provide test for errno_str()

Provide a unit test for errno_str(). Test that known and unknown error
numbers are handled correctly.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/lib/Makefile b/test/lib/Makefile
index 308c617..b13aaca 100644
--- a/test/lib/Makefile
+++ b/test/lib/Makefile
@@ -6,3 +6,4 @@
 obj-y += hexdump.o
 obj-y += lmb.o
 obj-y += string.o
+obj-$(CONFIG_ERRNO_STR) += test_errno_str.o