multi: context.Background() -> t.Context()

Use the new feature of Go 1.24, fix linter warnings.

This change was produced by:
 - running golangci-lint run --fix
 - sed 's/context.Background/t.Context/' -i `git grep -l context.Background | grep test.go`
 - manually fixing broken tests
 - itest, lntest: use ht.Context() where ht or hn is available
 - in HarnessNode.Stop() we keep using context.Background(), because it is
   called from a cleanup handler in which t.Context() is canceled already.

kvdb/etcd/readwrite_tx_test.go

@@ -4,7 +4,6 @@
 package etcd
 
 import (
-	"context"
 	"testing"
 
 	"github.com/btcsuite/btcwallet/walletdb"
@@ -16,7 +15,7 @@ func TestChangeDuringManualTx(t *testing.T) {
 
 	f := NewEtcdTestFixture(t)
 
-	db, err := newEtcdBackend(context.Background(), f.BackendConfig())
+	db, err := newEtcdBackend(t.Context(), f.BackendConfig())
 	require.NoError(t, err)
 
 	tx, err := db.BeginReadWriteTx()
@@ -44,7 +43,7 @@ func TestChangeDuringUpdate(t *testing.T) {
 
 	f := NewEtcdTestFixture(t)
 
-	db, err := newEtcdBackend(context.Background(), f.BackendConfig())
+	db, err := newEtcdBackend(t.Context(), f.BackendConfig())
 	require.NoError(t, err)
 
 	count := 0