Modify tests to suit new behaviour

Prevent the null argument test from running when
only MBEDTLS_MD_LIGHT is enabled.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/tests/suites/test_suite_md.data b/tests/suites/test_suite_md.data
index ccc7b10..15e3b99 100644
--- a/tests/suites/test_suite_md.data
+++ b/tests/suites/test_suite_md.data
@@ -3,6 +3,7 @@
 mbedtls_md_list:
 
 MD NULL/uninitialised arguments
+depends_on:MBEDTLS_MD_C
 md_null_args:
 
 Information on MD5