Change data file suffix for ott

Change the suffix of the data files searched in `mbedtls_test.py`
to `datax` as the generated files have this suffix.
diff --git a/tests/scripts/mbedtls_test.py b/tests/scripts/mbedtls_test.py
index 8e8a89b..f9e88cf 100755
--- a/tests/scripts/mbedtls_test.py
+++ b/tests/scripts/mbedtls_test.py
@@ -185,7 +185,7 @@
         binary_path = self.get_config_item('image_path')
         script_dir = os.path.split(os.path.abspath(__file__))[0]
         suite_name = os.path.splitext(os.path.basename(binary_path))[0]
-        data_file = ".".join((suite_name, 'data'))
+        data_file = ".".join((suite_name, 'datax'))
         data_file = os.path.join(script_dir, '..', 'mbedtls',
                                  suite_name, data_file)
         if os.path.exists(data_file):