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.

lncfg/dev.go

@@ -0,0 +1,23 @@
+//go:build !integration
+
+package lncfg
+
+import "time"
+
+// IsDevBuild returns a bool to indicate whether we are in a development
+// environment.
+//
+// NOTE: always return false here.
+func IsDevBuild() bool {
+	return false
+}
+
+// DevConfig specifies development configs used for production. This struct
+// should always remain empty.
+type DevConfig struct{}
+
+// ChannelReadyWait returns the config value, which is always 0 for production
+// build.
+func (d *DevConfig) ChannelReadyWait() time.Duration {
+	return 0
+}