Interoperability for test helpers

[aaronadamsCA]

We'd like to be able to use the setUpValidRequest() and setUpValidSession() test helpers in a browser environment. Right now they only work in a Node.js environment, and I haven't had any luck with polyfills.

Our use case is mocking Shopify app authentication in Storybook.

For now we've rolled our own interoperable helpers with two main changes.

We're using Jose for JWT signing:

return new SignJWT(payload)
  .setProtectedHeader({ alg: "HS256" })
  .sign(new TextEncoder().encode(CLIENT_SECRET));

And we're using btoa() (which is not deprecated in Node.js!) to encode the host:

const BASE64_HOST = btoa(SHOPIFY_HOST);

With these changes we get interop across multiple runtimes.