Grouping tests into one instance

Same-group tests share one disposable instance; independent groups run in parallel.

By default each test provisions its own instance. Give several tests the **same non-empty group (within a package) to run them sequentially inside one** shared instance — amortizing container start, and especially VM boot.

Grouped tests must agree on backend and image, and they **share OS state with no reset between them**, so use distinct paths/state per test. Independent groups run in parallel, throttled by --docker-jobs / --vmlab-jobs (see testlab flags).