`cabal-validate`: Steps are not granular enough, add groups of steps

Each `--step` in `cabal-validate` runs multiple test suites.

This makes it difficult to pinpoint which test suite caused a failure. This also makes rerunning tests (while fixing a failure) take longer than it needs to, because you may need to wait for unrelated test suites to finish. (The `--pattern` option usually helps, though.)

**Proposal:** Make steps run a single test suite instead of multiple, and introduce 'groups' to run multiple steps.

It might be nice if groups/steps have a nesting hierarchy, e.g. `build` builds everything, `build-lib` builds `Cabal`, `Cabal-hooks`, etc., and `build-Cabal` builds `Cabal` only.

Start with the cabal-validate entry point and trace how --step and --pattern select test suites. Map the existing multi-suite steps before deciding how individual steps and groups should be represented. Done means a single test suite can be run independently, groups can run multiple steps, and the hierarchy remains usable.

Written by the indexing model from the issue text.