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_integration.go

@@ -0,0 +1,26 @@
+//go:build integration
+
+package lncfg
+
+import "time"
+
+// IsDevBuild returns a bool to indicate whether we are in a development
+// environment.
+//
+// NOTE: always return true here.
+func IsDevBuild() bool {
+	return true
+}
+
+// DevConfig specifies configs used for integration tests. These configs can
+// only be used in tests and must NOT be exported for production usage.
+//
+//nolint:lll
+type DevConfig struct {
+	ProcessChannelReadyWait time.Duration `long:"processchannelreadywait" description:"Time to sleep before processing remote node's channel_ready message."`
+}
+
+// ChannelReadyWait returns the config value `ProcessChannelReadyWait`.
+func (d *DevConfig) ChannelReadyWait() time.Duration {
+	return d.ProcessChannelReadyWait
+}