Spec tests need to be version-locked on 1.5 branch

[schlessera]

Bug Description

We are currently always pulling the latest spec files from the [ampproject/amp-toolbox](https://github.com/ampproject/amp-toolbox project) to run the language-agnostic spec tests against our optimizer implementation.

This also happens in the separate 1.5 branch that we have open for bug fixes. However, while this causes the 1.5 branch to always run against the latest spec file tests, this branch does not get the latest updates to the Optimizer that would fix the code to make updated tests pass if behavior has changed upstream.

Expected Behaviour

The 1.5 branch (and any future release branch that is split off of develop) should be version-locked into a specific set of spec files to test against that is known to work.

Steps to reproduce

1. Check out commit 6f0ed6f
2. Run composer install within lib/optimizer folder
3. Run composer test within lib/optimizer folder

Screenshots

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Fixed by #4825 and #4826.