Publish test-utils as a package on NPM

[jonchurch]

Per discussion at the JS SIG meeting on 7/28/2021, it was decided to publish the test-utils as a package to improve workflow of contributors.

Needs a readme, and to be moved out into it's own folder (should it be moved into the opentelemetry-js repo?)

@rauno56 volunteered to work on this.

Refs: open-telemetry/opentelemetry-js#2085

[rauno56]

Thanks for doing the legwork of documenting it!

It's already in its own package here, but it just happens not to be published.

[rauno56]

Pumped into this one again, @dyladan. Please publish @opentelemetry/test-utils.