fix(appengine/standard-storage): refactor capybara testing

[glasnt]

Description

Resolves issues in updates to the testing stack.

• Ensure host authorization for all hosts while in development (solves for "Host not permitted")
• Use no-sandbox flag to ensure successful execution in Docker (prevents timeouts)
• Use NET::HTTP for external URL validation (prevents visit's host limitation)

Internal b/392885740

Would appreciate review and confirmation before applying to other samples in appengine/ space

Checklist

• Tests pass
• Lint pass: bundle exec rubocop
• Please merge this PR for me once it is approved.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• appengine/standard-storage/app.rb:24, tag gae_standard_storage_app

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[dazuma]

Nit: move this re-START tag down one line so we don't end up with two blank lines in the published sample.