Add URLPattern tests for Speculative Loading

[westonruter]

Feature Description

As @tunetheweb noted in #1143 (comment), it would be useful if we could check the validity of the URLPattern strings that we're constructing in the Speculative Loading plugin to ensure they are valid.

[swissspidy]

On that note, I was thinking about writing a JSON schema for speculation rules to ensure validity of that part too

[westonruter]

Yeah, that'll be important especially if the entire rules array is filterable, as the schema could be checked even at runtime.

[swissspidy]

Meanwhile, has this been covered by existing tests, e.g. in the core PR https://github.com/WordPress/wordpress-develop/pull/7860/files#diff-6577e99c0aa1f0d10c12d35bc4e071b7402cf833e0a65f6311b8fa505aac4012? It seems like it, so I am inclined to close this as complete.