Add MSW to testing package

[RobertBroersma]

@peterp I just started playing around with and this tools is totally awesome!

There's probably a lot more magic we can do for users, like providing a default file where users can define default mocks, but I think this is something we can figure out while we implement config extension and that kind of business.

Check this example for how to use: https://github.com/RobertBroersma/redwood-testing/blob/master/web/src/pages/BlogPostPage/BlogPostPage.test.js

It's good that this decouples us from Apollo.

It does add some tiny constraints, like you can't test anonymous GraphQL queries (or at least _I didn't manage to) but I think that could even be a positive as it enforces the better practice of naming your queries.

This should also Just Work™ with mutations but I haven't tested that yet. I got too excited and instantly opened this PR.

[peterp]

Woo hoo! @RobertBroersma this is amazing!

[peterp]

I'm wondering if there's a way to get rid of the "server" part here:

https://github.com/RobertBroersma/redwood-testing/blob/687d1b34f238cc6f627604c3f84d4b1dbd74e794/web/src/pages/BlogPostPage/BlogPostPage.test.js#L7-L20

Could we wrap graphql.query and graphql.mutation and then remove the server.use implementation detail for the user, maybe something like:

export const graphql = {
    query: (name, callback) => {
         return server.use(graphql.query(name, callback))
    },
    mutation: () => { /*...*/}
}