Hard to find failed test case

[Louson]

When running a test case, the output is checked with diff. It does not print the file in concern and so it's hard to know which file it is.

Example:

1. make some modification in test/basic/show/build-output.ref
2. run the test with ctest -VV -R show

We have the diff but not the file name

I have not found any option in diff to print the file name. Maybe we can wrap diff to print the file name in case of error.