Update TypeScript content in unit testing doc to use assertions

[davidwrede]

Currently, our unit testing topic uses the Mocha framework for unit testing Pulumi programs written in TypeScript.

A more idiomatic way would be to use assertions via Promises as shown in this blog post.

[stack72]

Any changes to this should be reflected back here - https://github.com/pulumi/examples/tree/master/testing-unit-ts :D