Add `mbedtls_` prefix to all public names in `helpers.h`

Adds the `mbedtls_` prefix to `test_result_t` and `test_info` and updates
any references to them. This is to follow the naming convention as these are
now declared in a public namespace.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/suites/test_suite_asn1parse.function b/tests/suites/test_suite_asn1parse.function
index 51cb3ca..75c3fd4 100644
--- a/tests/suites/test_suite_asn1parse.function
+++ b/tests/suites/test_suite_asn1parse.function
@@ -614,7 +614,7 @@
             cur = &head;
             while( *rest )
             {
-                ++test_info.step;
+                ++mbedtls_test_info.step;
                 TEST_ASSERT( cur != NULL );
                 TEST_EQUAL( cur->buf.tag, tag );
                 n = strtoul( rest, (char **) &rest, 0 );