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.
These are currently added implicitly by the compiler in the context of
`if`/`switch` expressions. In the future, there might be explicit
`then <expr>` statement useful for cases where one would like to add
more than one statement in the branch, to mark what value to actually
use.
See https://forums.swift.org/t/pitch-multi-statement-if-switch-do-expressions/68443
