Expand the .gitignore file

[desrosj]

This adds several patterns to the .gitignore file to prevent accidentally committing files related to contributing through version control.

The wp-test-runner directory is also being added as the default directory for running tests locally.

[github-actions]

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Co-authored-by: desrosj <desrosj@git.wordpress.org>
Co-authored-by: zunaid321 <zunaid321@git.wordpress.org>
Co-authored-by: kittenkamala <amykamala@git.wordpress.org>

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