config UPDATE add config replace test

Added test for replacing configuration data. Renamed configuration setup
API calls. Added loading JSON from files.
diff --git a/tests/test_truststore.c b/tests/test_truststore.c
index aa5af9f..5b38658 100644
--- a/tests/test_truststore.c
+++ b/tests/test_truststore.c
@@ -222,7 +222,7 @@
     assert_int_equal(ret, 0);
 
     /* configure the server based on the data */
-    ret = nc_server_config_setup(tree);
+    ret = nc_server_config_setup_diff(tree);
     assert_int_equal(ret, 0);
 
     /* initialize client */