Allow TestEngines to be completely dynamic

[sbrannen]

Status Quo

It is currently impossible to create a custom TestEngine that is completely dynamic in nature.

The reason is that a completely dynamic engine will not have any statically registered children and will therefore get summarily pruned by org.junit.platform.launcher.core.Root.pruneEmptyTestEngines().

Deliverables

• Allow test engines to be completely dynamic (i.e., don't prune engines without children).

[sbrannen]

@junit-team/junit-lambda, let's discuss the merits of this proposal.

See also #366, which was the impetus for this proposal. Pay special attention to the fact that one has to create a dummy container TestDescriptor and add it to the engine in order to prevent the dynamic engine from getting pruned. See the inline comments in DynamicTestEngine.discover() in 47d20a7 for details.

[sbrannen]

in progress