Rollup merge of #147806 - samueltardieu:chores/gitignore-test-dashboard, r=jieyouxu,kobzol

Ignore test-dashboard related files

After bors merge 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`.

Author: matthiaskrgr