Altitude-specific labels for subset inputs/outputs

To run a subset of tests, you pass the returned subset list into your test runner for execution.

Each test runner has a distinct method for running a specific set of tests, resulting in different 'altitudes' of filtering. For example, some test runners only let you pass in a list of files to run, others support filtering by class, while some support filtering by test case or method.

Based on the test runner specified in launchable subset, the CLI automatically outputs a list of tests using the hierarchy level supported by that test runner.

For example, Maven supports filtering by class, so we say that Maven's subset altitude is class. A test item for Maven is equivalent to a class. Test results captured using launchable record tests for Maven will include both class and test case identifiers, but the test item output of launchable subset will include a list of classes.

Until now, we've referred to these as test items in the web app. With this update, they are now correctly labeled as files, classes, test cases, targets, etc.