Add e2e test for maintenance mode and other edge cases

[swissspidy]

In this PR:

• A test for the fatal error handler
• A test for maintenance mode
• A test for the famous 5 minute install
  • Aims to detect issues such as https://core.trac.wordpress.org/ticket/61312#comment:29 / https://wordpress.slack.com/archives/core/p1718614091060489

In the future:

• Recovery mode flow
  • Requires sending an email or another way of getting the recovery mode link
• Test maintenance mode or fatal error handler with a different locale
  • For Parse Accept-Language header in wp_load_translations_early #6623

Trac ticket: https://core.trac.wordpress.org/ticket/61240

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[github-actions]

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[swissspidy]

Committed in https://core.trac.wordpress.org/changeset/58430