run-test: restore the -i prompt by write .err before checking if it exists The `--interactive` flag workis by overwriting the original test file by its `.err` version. So we need to write it before calling `self.fail`. Otherwise the `.err` file does not exists and `--interactive` is ignored. We can move that block code around because it is dedicated to write changed output and we moves it in the try-except dedicated to handling changed output. Note that the flog is still badly broken after this change. But at least it crash instead of being ignored.