Test failure should give test filename, not output directory name

While running `dotty.tools.dotc.CompilationTests.runAll` I got:
```
    Compilation failed for: '../out/runAll/run/case-class-23'                  
```
But I don't care about the name of the output directory, I'm interested in the name of the source file(s), in this case `tests/run/case-class-23.scala`. I'm also interested in how to rerun just this test.