Disable invalid argument MD test case

With the removal of the MD handle from the MD context, it's a precondition
for any MD API outside of mbedtls_md_init() and mbedtls_md_setup() that
the MD context has been successfully setup by precisely those functions
beforehand, and hence must be bound to the single enabled valid MD handle.

tests/suites/test_suite_md.function

@@ -39,7 +39,7 @@ exit:
 }
 /* END_CASE */
 
-/* BEGIN_CASE */
+/* BEGIN_CASE depends_on:!MBEDTLS_MD_SINGLE_HASH */
 void md_null_args(  )
 {
     mbedtls_md_context_t ctx;