Remote Runner / PostMessge api

[flashdesignory]

This pr introduces Remote Runner workloads.
Workloads that are local OR that point to a remote url (when running Speedometer locally), can communicate and run their own tests via PostMessage.

Basic flow:

1. SuiteRunner loads the iframe of the local or remote workload.
2. SuiteRunner waits for the "app-ready" message from the workload.
3. Workload sends "app-ready" message, once its able to listen and receive a command to start the tests.
4. SuiteRunner sends the "benchmark-suite" command to start the testing (with an optional suite name).
5. Workload runs through the test suite and collects performance metrics.
6. After each completed test step it sends a "step-complete" message, to update progress in speedometer.
7. With the "suite-complete" message, the workload sends the metrics along.
8. Speedometer receives metrics.

Notes

• TestRunner and TestInvoker files are installed as dependencies in the news-site-next workload, to ensure they stay synced with updates.

[bgrins]

@flashdesignory I don't see the following logic used in the main runner. Since it also reads params from the URL, would it be possible to share some code here instead?

[bgrins]

I've shared params.mjs in the other example 728aeab

[flashdesignory]

we could - the params.mjs is slightly different, but I can take a look at both and see if I can clean up and reuse

[bgrins]

Now that this has mostly incorporated #459 (I'd still like to see the params sharing) I think it's going to be easier to review & hopefully get landed for testing @rniwa @julienw