Use TEST_EQUAL in one more place in test_suite_md

The only remaining occurrences of TEST_ASSERT are now pointer comparison,
to NULL or to a reference md_info. That is, the output of the following
command is empty:

    grep TEST_ASSERT tests/suites/test_suite_md.function |
        egrep -v '= NULL|== md_info|md_info =='

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/suites/test_suite_md.function b/tests/suites/test_suite_md.function
index 0828fd7..c1cf8ec 100644
--- a/tests/suites/test_suite_md.function
+++ b/tests/suites/test_suite_md.function
@@ -50,7 +50,7 @@
     mbedtls_md_init(&ctx);
 
     TEST_EQUAL(0, mbedtls_md_get_size(NULL));
-    TEST_ASSERT(mbedtls_md_get_type(NULL) == MBEDTLS_MD_NONE);
+    TEST_EQUAL(mbedtls_md_get_type(NULL), MBEDTLS_MD_NONE);
     TEST_ASSERT(mbedtls_md_get_name(NULL) == NULL);
 
     TEST_ASSERT(mbedtls_md_info_from_string(NULL) == NULL);