buildifier_test does not complain when passix fix as a mode

When passing fix to the lint_mode or mode to buildifier_test, Bazel does not complain.
This is not nice from a user point of view.

```
buildifier_test(
name = "buildifier",
size = "small",
timeout = "short",
lint_mode = "fix",
mode = "fix",
no_sandbox = True,
workspace = "//:WORKSPACE",
)
```

I tried with buildtools 6.4.0 and Bazel 7.0.0

Start with the buildifier_test rule and reproduce the issue using the provided BUILD snippet with buildtools 6.4.0 and Bazel 7.0.0. Trace how lint_mode and mode are validated; done means passing "fix" causes Bazel to report a configuration error instead of being accepted.

Written by the indexing model from the issue text.