test: Add the beginnings of some string tests

There are quite a few string functions in U-Boot with no tests. Make a
start by adding a test for strtoul().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/Makefile b/test/Makefile
index 2971d0d..bab8f1a 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -8,6 +8,7 @@
 obj-$(CONFIG_SANDBOX) += command_ut.o
 obj-$(CONFIG_SANDBOX) += compression.o
 obj-$(CONFIG_SANDBOX) += print_ut.o
+obj-$(CONFIG_SANDBOX) += str_ut.o
 obj-$(CONFIG_UT_TIME) += time_ut.o
 obj-$(CONFIG_UT_UNICODE) += unicode_ut.o
 obj-y += log/