multi: add golang 1.17 compatible build tags

With go 1.17 a change to the build flags was implemented:
https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

The formatter now automatically adds the forward-compatible build tag
format and the linter checks for them, so we need to include them in our
code.

lntest/bitcoind.go

@@ -1,5 +1,5 @@
-// +build bitcoind
-// +build !notxindex
+//go:build bitcoind && !notxindex
+// +build bitcoind,!notxindex
 
 package lntest
 

lntest/bitcoind_common.go

@@ -1,3 +1,4 @@
+//go:build bitcoind
 // +build bitcoind
 
 package lntest

lntest/bitcoind_notxindex.go

@@ -1,5 +1,5 @@
-// +build bitcoind
-// +build notxindex
+//go:build bitcoind && notxindex
+// +build bitcoind,notxindex
 
 package lntest
 

lntest/btcd.go

@@ -1,3 +1,4 @@
+//go:build !bitcoind && !neutrino
 // +build !bitcoind,!neutrino
 
 package lntest

lntest/itest/lnd_etcd_failover_test.go

@@ -1,3 +1,4 @@
+//go:build kvdb_etcd
 // +build kvdb_etcd
 
 package itest

lntest/itest/lnd_max_channel_size_test.go

@@ -1,3 +1,4 @@
+//go:build rpctest
 // +build rpctest
 
 package itest

lntest/itest/lnd_no_etcd_dummy_failover_test.go

@@ -1,3 +1,4 @@
+//go:build !kvdb_etcd
 // +build !kvdb_etcd
 
 package itest

lntest/itest/lnd_test_list_off_test.go

@@ -1,3 +1,4 @@
+//go:build !rpctest
 // +build !rpctest
 
 package itest

lntest/itest/lnd_test_list_on_test.go

@@ -1,3 +1,4 @@
+//go:build rpctest
 // +build rpctest
 
 package itest

lntest/neutrino.go

@@ -1,3 +1,4 @@
+//go:build neutrino
 // +build neutrino
 
 package lntest

lntest/timeouts.go

@@ -1,3 +1,4 @@
+//go:build !darwin && !kvdb_etcd && !kvdb_postgres
 // +build !darwin,!kvdb_etcd,!kvdb_postgres
 
 package lntest

lntest/timeouts_darwin.go

@@ -1,3 +1,4 @@
+//go:build darwin && !kvdb_etcd && !kvdb_postgres
 // +build darwin,!kvdb_etcd,!kvdb_postgres
 
 package lntest

lntest/timeouts_remote_db.go

@@ -1,3 +1,4 @@
+//go:build kvdb_etcd || kvdb_postgres
 // +build kvdb_etcd kvdb_postgres
 
 package lntest