multi: add itest to check race between channel_ready and update_add_htlc

This commit adds a new itest case to check the race condition found in
issue #7401. In order to control the funding manager's state, a new dev
config for the funding manager is introduced to specify a duration we
should hold before processing remote node's channel_ready message.

A new development config, `DevConfig` is introduced in `lncfg` and will
only have effect if built with flag `integration`. This can also be
extended for future integration tests if more dev-only flags are needed.

config.go

@@ -490,6 +490,10 @@ type Config struct {
 
 	// Estimator is used to estimate routing probabilities.
 	Estimator routing.Estimator
+
+	// Dev specifies configs used for integration tests, which is always
+	// empty if not built with `integration` flag.
+	Dev *lncfg.DevConfig `group:"dev" namespace:"dev"`
 }
 
 // GRPCConfig holds the configuration options for the gRPC server.