Add accessor helpers for mbedtls_test_info

Step one of being able to control access to mbedtls_test_info with
a mutex.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/suites/test_suite_dhm.function b/tests/suites/test_suite_dhm.function
index e6f75de..2090594 100644
--- a/tests/suites/test_suite_dhm.function
+++ b/tests/suites/test_suite_dhm.function
@@ -31,7 +31,7 @@
     int ok = 0;
     mbedtls_mpi_init(&actual);
 
-    ++mbedtls_test_info.step;
+    mbedtls_test_increment_step();
 
     TEST_ASSERT(size >= *offset + 2);
     n = (buffer[*offset] << 8) | buffer[*offset + 1];