Allow integration tests to run against built code

[lobsterkatie]

As part of our work on v7, we're revamping our bundling process, both to standardize it and speed it up, and to allow us to create more variations of out our packages (with and without debug statements, ES5 and ES6, cjs and esm, bundled and via npm, minified, etc).

As a way to verify that all of our various builds work correctly (and as a way to identify errors which might not show up in all variations), we want to adapt our integration tests to run against built files as well as source files. (Right now, our TS source is compiled on the fly each time we run tests.)

To be more specific, the goal is to have a core set of integration tests, runnable in two frameworks: one which uses a pre-built bundle, which can be passed in, and one which builds with webpack or rollup, using different flavors of our npm package.