itest+lntest: add flakePaymentStreamReturnEarly

Fix the flake found in the `testRelayingBlindedError` and documents the
flake found in other tests in one place.

itest/lnd_route_blinding_test.go

@@ -667,6 +667,10 @@ func testRelayingBlindedError(ht *lntest.HarnessTest) {
 	// so that she can't forward the payment to Dave.
 	testCase.drainCarolLiquidity(false)
 
+	// NOTE: The above draining requires Carol to send a payment, which may
+	// create extra events, causing the `AssertHtlcEvents` to fail below.
+	flakePaymentStreamReturnEarly()
+
 	// Subscribe to Carol's HTLC events so that we can observe the payment
 	// coming in.
 	carolEvents := testCase.carol.RPC.SubscribeHtlcEvents()