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.