Consider defining the driver workload in its own file

[p-mongo]

@jmikola suggests defining driver workloads (which now use the unified test format, which comes with a validator) in a separate file, so that they can be easily validated:

Looking at the test files in #98, it looks like unified test format files (schemaVersion 1.0, so I assume no format changes) are nested within other files. Can this be changed to separate them out so we can actually perform schema validation on them? For example, the workload executor could point to a filename (or base name, so drivers can decide to use YAML or JSON).