tests: test_xpay_fake_channeld is skipped rather than xfail

[nGoline]

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 xpay: fakenet test: move funds when payment succeeds #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 Askrene and xpay improve #9150, and updating the daemon to move liquidity exposes the older xpay bug in xpay: fakenet test: move funds when payment succeeds #9282.

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