Adds GIT_AUTHOR_NAME to the test env

[arcanis]

What's the problem this PR addresses?

The test env doesn't automatically setup the git committer name / email. I noticed it when working on yarn init, as the test was passing but I didn't understand why until I dig deeper (turns out we don't pass the strict flag when calling the git commands; I don't remember whether it's an oversight or not).

How did you fix it?

Automatically setup the git committer name / email. Should a test want to avoid that (for example to explicitly test that a command works without), they should override it manually.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.