test: env: Add test for verifying env attrs

Add a test of the env_attr_lookup() function.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/env/Makefile b/test/env/Makefile
index 59b38e9..5168bcb 100644
--- a/test/env/Makefile
+++ b/test/env/Makefile
@@ -5,3 +5,4 @@
 #
 
 obj-y += cmd_ut_env.o
+obj-y += attr.o