fix(functional-tests): failures due to rate limiting in coupon tests

[Trinaa]

Because

• coupon tests are failing in stage due to IP rate limiting

This pull request

• adds signin as a pre-requisite to coupon tests
  • since all subscription tests now require signin, the steps have been encapsulated in an automatic fixture to reduce code duplication (subscriptionFixtures.ts)
  • without signin rate limiting is measured per IP, with signin it is measured per account. Since tests have isolated accounts, signing in should add the resilience we need.

Issue that this pull request solves

Closes # FXA-10045

Checklist

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• [x] My commit is GPG signed.
• [x] If applicable, I have modified or added tests which pass locally.
• [ ] I have added necessary documentation (if appropriate).
• [ ] I have verified that my changes render correctly in RTL (if appropriate).

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