FS: port q-s-m tests to new infrastructure

[mrBliss]

Port the existing q-s-m tests to use the high-level interface described in https://github.com/advancedtelematic/quickcheck-state-machine/pull/355. See http://www.well-typed.com/blog/2019/01/qsm-in-depth/ for more details. As the FS tests have references (file handles and paths), Test.StateMachine.Lockstep.NAry has to be used instead of Test.StateMachine.Lockstep.Simple.

This interface will likely need some changes before we can adopt it. It might be a good idea to make a local copy of that q-s-m infrastructure so that we can quickly modify it instead of having to submit PRs against the upstream repo and wait for them to get merged.

Related: input-output-hk/ouroboros-consensus#620.

[mrBliss]

Some things I already discovered were missing from the new high-level interface:

• Preconditions
• Per-test setup, e.g., create an IORef. Idea: introduce initEnv and pass its result to runReal and cleanup
• A way to run a specific Commands
• Documentation :grin: