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.

itest/lnd_funding_test.go

@@ -1,7 +1,6 @@
 package itest
 
 import (
-	"context"
 	"fmt"
 	"testing"
 
@@ -1164,7 +1163,7 @@ func ensurePolicy(ht *lntest.HarnessTest, alice, peer *node.HarnessNode,
 
 	channel := ht.AssertChannelExists(peer, chanPoint)
 	policy, err := peer.RPC.LN.GetChanInfo(
-		context.Background(), &lnrpc.ChanInfoRequest{
+		ht.Context(), &lnrpc.ChanInfoRequest{
 			ChanId: channel.ChanId,
 		},
 	)