tests: test_xpay_fake_channeld is skipped rather than xfail

Follow-up from #9150.

`test_xpay_fake_channeld` is marked `@pytest.mark.skip(reason="channeld_fakenet needs updating")`. A plain `skip` stays silent once the underlying problem is fixed, so the test will sit disabled indefinitely.

Two things:

1. Use `@pytest.mark.xfail(strict=True)` so it fails loudly when it starts passing.
2. Reference #9282 in the reason. The test currently fails for at least two reasons: channeld_fakenet simulates fixed per-channel liquidity, which is incompatible with the impressions model added in #9150, and updating the daemon to move liquidity exposes the older xpay bug in #9282.

Related: the same PR added an `askrene_age('xpay', 1)` call inside the test loop as a workaround for impression buildup, see #9375.