Set test footnote ID for GLFM snapshot examples

What does this MR do and why?

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

Sets GITLAB_TEST_FOOTNOTE_ID during GLFM snapshot generation to prevents random IDs in footnotes.

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)

This is a follow-up to Allow override of random footnote ID (!90959 - merged), please see that MR for a detailed explanation.

How to set up and validate locally

1. Re-run scripts/glfm/update-example-snapshots.rb
2. Observe that the 07_01__gitlab_specific_markdown__footnotes__001 example in glfm_specification/example_snapshots/html.yml has consistent, non-random sub-entries.

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