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/target_test.function b/tests/suites/target_test.function
index 8354b96..7866dcf 100644
--- a/tests/suites/target_test.function
+++ b/tests/suites/target_test.function
@@ -384,8 +384,8 @@
     while ( 1 )
     {
         ret = 0;
-        test_info.result = TEST_RESULT_SUCCESS;
-        test_info.step = (unsigned long)( -1 );
+        mbedtls_test_info.result = MBEDTLS_TEST_RESULT_SUCCESS;
+        mbedtls_test_info.step = (unsigned long)( -1 );
         data_len = 0;
 
         data = receive_data( &data_len );
@@ -443,7 +443,7 @@
         if ( ret )
             send_failure( ret );
         else
-            send_status( test_info.result );
+            send_status( mbedtls_test_info.result );
     }
     return( 0 );
 }