Implement NIE_20 (event finish before step, teardown at scenario end)

Per #203, `NIE_20` is currently not implemented (only `NIE_23` and `NIE_33` are, see #197 and `ExecutionMode.NON_INTERRUPTIVE`'s `behaviorTypes` whitelist).

| Behavior | Event Start | Event Start Finish | Tear Down |
| - | - | - | - |
| NIE_20 | Before Step | Before Step | latest scenario end |

The event must finish starting before the step runs (like `NIE_22`/`NIE_23`), but tear-down is deferred until the end of the scenario rather than happening right after a specific step.

See the shared behavior-code documentation in [README.md#non-interruptive-execution-mode](https://github.com/adobe/phased-testing/blob/main/README.md#non-interruptive-execution-mode) and [README.md#wrapping-an-event-around-a-step](https://github.com/adobe/phased-testing/blob/main/README.md#wrapping-an-event-around-a-step).

Start with the non-interruptive execution sections in README.md, then inspect ExecutionMode.NON_INTERRUPTIVE's behaviorTypes whitelist and the existing NIE_22/NIE_23 implementation referenced by the issue. Implement NIE_20 so the event finishes starting before the step and teardown is deferred to the latest scenario end, and verify the behavior against the documented table.

Written by the indexing model from the issue text.