[SSR] Add test for decorators within a mixin class

[nolanlawson]

Follow-up to https://github.com/salesforce/lwc/pull/4942/files#r1858956004

Add a fixture test in engine-server/src/_tests__/fixtures like this:

export const FancyMixin = Class => {
    return class extends Class {
      @api foo
    }
}

We should test that the foo prop is actually exposed.