Add artifacts to test runs

To facilitate the below two use-cases, we should add the ability to upload "artifacts" to completed test runs:

1. When a test fails, we want to upload the log files of each node involved.
2. When submitting a performance test, it's important to be able to upload or link to the graph output of the test.

/cc @matthewvon @evanmcc @joedevivo @cmeiklejohn