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.

itest/list_on_test.go

@@ -546,4 +546,8 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "utxo selection funding",
 		TestFunc: testChannelUtxoSelection,
 	},
+	{
+		Name:     "update pending open channels",
+		TestFunc: testUpdateOnPendingOpenChannels,
+	},
 }