lntest/mpp_test: ensure ctx.alice and ctx.bob are shutdown

Not doing so prevents consecutive subtests from starting properly.

lntest/itest/lnd_mpp_test.go

@@ -344,6 +344,8 @@ func (c *mppTestContext) closeChannels() {
 }
 
 func (c *mppTestContext) shutdownNodes() {
+	shutdownAndAssert(c.net, c.t, c.alice)
+	shutdownAndAssert(c.net, c.t, c.bob)
 	shutdownAndAssert(c.net, c.t, c.carol)
 	shutdownAndAssert(c.net, c.t, c.dave)
 	shutdownAndAssert(c.net, c.t, c.eve)