Enhance support for executing a single test method in a JUnit class

Enhance support for executing a single test method in a JUnit class: including parameterized test methods.

https://junit.org/junit5/docs/current/api/org.junit.platform.engine/org/junit/platform/engine/discovery/DiscoverySelectors.html offers a set of "selectMethod()" factory methods - and there's also a general-purpose "selectUniqueId()" selector available.