This collapses all generated test QL sources into a single one per
directory, using query predicates to run the different tests.
This should improve the time required to run generated tests.
When skipping bodies in library code, we lose the information whether a
body was originally present. This can be important, for example when
determining whether a trait method has a default implementation.
With this change that information can be recovered via the
`hasImplementation` predicate.
