Show diff when run shelltest failed

Author: owent

functional/configuration/run_test.sh

@@ -15,5 +15,5 @@ export EXAMPLE_BIN_DIR="${BUILD_DIR}/examples/configuration/"
 # Make sure `example_yaml` is in the path
 export PATH=${PATH}:${EXAMPLE_BIN_DIR}
 
-shelltest ./shelltests
+shelltest ./shelltests --diff-cmd="diff -u --color=always"