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/walletdb_interface_test.go

@@ -4,7 +4,6 @@
 package etcd
 
 import (
-	"context"
 	"testing"
 
 	"github.com/btcsuite/btcwallet/walletdb/walletdbtest"
@@ -15,5 +14,5 @@ import (
 func TestWalletDBInterface(t *testing.T) {
 	f := NewEtcdTestFixture(t)
 	cfg := f.BackendConfig()
-	walletdbtest.TestInterface(t, dbType, context.Background(), &cfg)
+	walletdbtest.TestInterface(t, dbType, t.Context(), &cfg)
 }