Add configuration parsing to transformer spec test runner

[schlessera]

Feature description

The language agnostic spec tests in the amp-toolbox package use a leading HTML comment to configure individual tests: https://github.com/ampproject/amp-toolbox/blob/bb242b62266a71c777437d199161ba850ddab53b/packages/optimizer/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/input.html#L1-L6

This is now being used for a few tests that we are running as well, so we need to implement the same thing.

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Acceptance criteria

• Leading HTML comments with arguments should properly configure the transformer spec tests.

Implementation brief

• Check if the input document starts with an HTML comment.
• If yes, fetch the HTML comment and strip it from the input file.
• Use json_decode() on the text content of the comment.
• The resulting JSON document represents the options that should be passed into the transformer(s).

Note: This might require mapping from Node.js options to PHP options if these are not the same. Alternatively, we might consider adapting the options to keep them in sync with the Node.js version.

QA testing instructions

Demo

Changelog entry

[westonruter]

Please prioritize this so that builds will pass again.

[pierlon]

I don't think can be QA'd since it pertains to parsing of Optimizer spec tests, unless I'm mistaken @schlessera?