[heycam/webidl] Require tests for PR/issues? (#268)

This has been suggested by @annevk and @Ms2ger.

Opening an issue here to come up with a better understanding of how we're testing WebIDL so far, what requires tests would precisely entail, and drafting some text around this to put in the contributing doc.

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https://github.com/heycam/webidl/issues/268