Add config file to utest to ensure UNITY_INCLUDE_CONFIG_H is always

defined.
This is required for Unity in order to ensure that unity_config.h is
included in builds.

frameworks/utest/mbed_lib.json

@@ -0,0 +1,4 @@
+{
+    "name": "utest",
+    "macros": ["UNITY_INCLUDE_CONFIG_H"]
+}