I can't find any dedicated tests that actually exercises the stability
gating (via `-Z unstable-options`) of print requests, so here's a
dedicated one.
I coalesced `tests/ui/feature-gates/feature-gate-print-check-cfg.rs`
into this test, because AFAICT that print request is not feature gated,
but only `-Z unstable-options`-gated just like other unstable print
requests.