build: Make tests depend on the helper required by one of them
This allows doing:
meson setup _build
meson test -C _build
without an intervening meson compile
step. Previously, this would have
failed in tests/testing
because testing-helper
was never compiled.
For a project this small, there's no real need to distinguish precisely which tests need the helper: we can have a simpler build by just assuming that they all do, like Autotools would.
/cc @denittis