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/go.mod

@@ -15,7 +15,6 @@ require (
 	go.etcd.io/etcd/client/pkg/v3 v3.5.12
 	go.etcd.io/etcd/client/v3 v3.5.12
 	go.etcd.io/etcd/server/v3 v3.5.12
-	golang.org/x/net v0.39.0
 	modernc.org/sqlite v1.29.10
 )
 
@@ -118,6 +117,7 @@ require (
 	golang.org/x/crypto v0.37.0 // indirect
 	golang.org/x/exp v0.0.0-20240325151524-a685a6edb6d8 // indirect
 	golang.org/x/mod v0.17.0 // indirect
+	golang.org/x/net v0.39.0 // indirect
 	golang.org/x/sync v0.13.0 // indirect
 	golang.org/x/sys v0.32.0 // indirect
 	golang.org/x/text v0.24.0 // indirect