The `cpp-unit-test` and `xctest` plugins should have consistent names and ids

These plugins both basically play the same role, which is to add support for a particular kind of native test framework _and_ apply a convention that the project has a unit test suite that uses the framework. Their class names and ids should reflect this and do so consistently.

Start by locating the implementations and plugin IDs for the `cpp-unit-test` and `xctest` plugins. Compare their class names and IDs, then check existing references and tests before deciding on consistent naming; the work is done when both plugins follow the same naming pattern without breaking their references.

Written by the indexing model from the issue text.