This adds a new `MonadConc` primitive, `_concNoTest`, which is (for
all non-test implementations) the identity function. For test
implementations, it is understood as "this action is completely safe
under all schedules, so just execute it all at once and don't consider
any internal interleavings." It is not required to be deterministic,
merely to never fail.
Actions annotated with `_concNoTest` will show up as one step in the
trace, and new `Failure` and `ThreadAction` values have been added.
- Tweak order in which schedules are explored to look at simple,
but not too simple, cases first.
- Port simplicity logic from shrinking to dupe elimination.
