Ignore test-dashboard related files

After bors merges a PR, a message may be posted from the CI onto the PR
comment thread directing the user to use the following commands to look
at the test differences:

```
cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard [SOMEID] --output-dir test-dashboard
open test-dashboard/index.html
```

This command creates the `.citool-cache` and `test-dashboard`
directories, whose contents should not enter the repository and can be
ignored by `git`.