[RTG][Elaboration] Improve how tests are matched with targets

[maerhart]

• Tests can now carry a string identifying the original test name because a test can be cloned to test multiple targets. This is useful for result reporting
• Tests can now specify a fixed target. Before elaboration it acts as a restriction to only be elaborated against that target, after elaboration it specifies for which target the test was elaborated.
• The body of a target is not copied into the tests anymore. This allows us to materialize operations that are only legal in targets and keep them until later in the pipeline.
• The test <-> target matching now supports a kind of 'subtyping'
• Remove the special casing for tests without requirements.