Child-side entry point that prints, as a JSON array, the source location
of every leaf test's it/prop/specify call site, then exits. Each
element is a TestLocation (test id, source file, line).
A leaf whose CallStack is empty (it carries no recorded frame), or whose
source file does not parse as a relative path, is omitted: it cannot be
mapped to a source line, so the diff-scoped runner has no use for it.
The most-recent (head) frame of the CallStack is the
it/prop/specify call site, because those combinators use
withFrozenCallStack to fix the user's call site as the top frame.