Archived test files and multiple runs simultaneously

Add support for dumping test files in a zipped format from the APITest executable (APITest test --dump-files).

Add support for retrieving archived test files from server with new api_test/{id}/files endpoint.

Add support for running more than one test simultaneously (or rather coincidentally) to the server. Previously you could request tests in rapid fire, but test files were all generated in the same folder so this was not really supported. Even now, the number of concurrently running tests is 1, but async operations from preparing or tearing down tests will not step on the toes of other test runs.