invoices: add TestSpontaneousAmpPayment

Adds a set of test cases that exercise the spontaneous AMP payment flow
with valid and invalid reconstructions, as well as with single and
multiple HTLCs. This also asserts that spontaneous AMP is gated behind
the existing AcceptKeysend flag.

invoices/resolution_result.go

@@ -180,6 +180,23 @@ func (f FailResolutionResult) FailureString() string {
 	}
 }
 
+// IsSetFailure returns true if this failure should result in the entire HTLC
+// set being failed with the same result.
+func (f FailResolutionResult) IsSetFailure() bool {
+	switch f {
+	case
+		ResultAmpReconstruction,
+		ResultHtlcSetTotalTooLow,
+		ResultHtlcSetTotalMismatch,
+		ResultHtlcSetOverpayment:
+
+		return true
+
+	default:
+		return false
+	}
+}
+
 // SettleResolutionResult provides metadata which about a htlc that was failed
 // by the registry. It can be used to take custom actions on resolution of the
 // htlc.