Clean up lineage/txtar testing framework

[sdboyer]

The test framework itself is also in a messy, halfway state. In retrospect, i realize what i was really trying to do was graft a subtest layer on top of the txtar-specific test runner. The framework itself already creates a subtest per txtar, but i was effectively trying to make each provided subtest executed from there be able to construct its own subtests in a way that's basically similar to stdlib *testing.T.Run.

Given that the central object in the framework is the lineage, this is crucial to allow for more standard helpers that, for example, allow a particular test case to be executed across all schemas in a lineage, and keeping each subtest's output therein tidily namespaced.

Originally posted by @sdboyer in #90 (comment)

This messiness needs to be resolved, in the direction of actually making what's in the lineage test harness shaped like subtesting.

I can imagine that we may want to re-copy the original upstream CueTxTar testing framework again to create another, less one-lineage-centric version of this framework. But for now, we already have five packages within encoding, and i think this approach is most productive for testing at least all of those, and probably also the CLI.