Multiple selections in test UI run individually/isolated

Originally raised at redhat-developer/vscode-java#4338.

Observed behaviour: multiple methods under a Java test class (or multiple test classes under a package) selected in the Tests sidebar tree seem to run significantly slower than when just selecting the single parent class (or package).

This seems to be because each selection does a full setup/teardown (i.e. selecting a class does one setup/teardown cycle, before the first and after the last method; selecting four methods from that class does four cycles, one around each method). For projects like Spring web applications and their unit tests, the application is recreated unnecessarily for each test, and for large applications this causes a significant slowdown. Repeating failed unit tests from a previous run doesn't seem to be affected in this way.

My understanding is that JUnit at least doesn't provide much of a test selection interface, so I'm assuming there's a custom runner in play in order to target the selected or previously-failed tests in the first place.

Start by reproducing the slowdown from the Tests sidebar with multiple methods and classes, comparing it with selecting the parent class or package. Trace the custom selection and execution path, paying attention to where setup and teardown are invoked. Done means multiple selections share the appropriate setup/teardown cycle without changing reruns of previously failed tests.