net/netip: test that Addr.AsSlice is inlineable

net/netip: test that Addr.AsSlice is inlineable

CL 584995 made Addr.AsSlice inlineable as a side effect. Make sure we
don't regress.

Updates #56136

Change-Id: Ib5f77a430db66ffe45c4cbb264da7a401945fec9
Reviewed-on: https://go-review.googlesource.com/c/go/+/585057
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>