itest: test all combos of rbf close and taproot chans

In this commit, we test all the combinations of rbf close and taproot
chans. This ensures that the downgrade logic works properly.

Along the way we refactor the tests slightly, and also split them up, as
running all the combos back to back mines more than 50 blocks in a test,
which triggers an error in the itest sanity checks.

lntest/node/config.go

@@ -68,6 +68,12 @@ var (
 		"--protocol.simple-taproot-chans",
 	}
 
+	// CfgRbfCoopClose specifies the config used to create a node that
+	// supports the new RBF close protocol.
+	CfgRbfClose = []string{
+		"--protocol.rbf-coop-close",
+	}
+
 	// CfgZeroConf specifies the config used to create a node that uses the
 	// zero-conf channel feature.
 	CfgZeroConf = []string{