Allow override of random footnote ID

What does this MR do and why?

Related: Tracking Issue: Implement GLFM scripts per the ... (#361241)

We allow overriding the randomness of footnote IDs with a static value from GITLAB_TEST_FOOTNOTE_ID.

This allows stable generation of example HTML during GLFM Snapshot Testing (https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#markdown-snapshot-testing), and reduces the need for normalization of the example HTML (https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#normalization)

Screenshots

How to set up and validate locally

• This is only intended to be used while updating GLFM example snapshots (see docs: https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#update-example-snapshotsrb-script).
• But, you can set the GITLAB_TEST_FOOTNOTE_ID=42 environment variable prior to running the GDK, and verify that footnotes have the specified ID (see screenshot above).

MR acceptance checklist

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