Fix typo in use of test_name

[michaelquinn32]

It should start with a dot.

Thanks for contributing to DoubleML. Before submitting a PR, please take a look at our contribution guidelines. Additionally, please fill out the PR checklist below.

Description

Please describe all changes and additions. In addition, you may want to comment on the diff in GitHub.

Reference to Issues or PRs

Add references to related issues or PRs here.

Comments

Here you can add further comments. You can also delete this section, if it is not necessary.

PR Checklist

Please fill out this PR checklist (see our contributing guidelines for details).

• [ X] The title of the pull request summarizes the changes made.
• [X ] The PR contains a detailed description of all changes and additions.
• [X ] References to related issues or PRs are added.
• [X ] The code passes R CMD check and all (unit) tests (see our contributing guidelines for details).
• [X ] Enhancements or new feature are equipped with unit tests.
• [X ] The changes adhere to the "mlr-style" standards (see our contributing guidelines for details).