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_signer_test.go

@@ -2,7 +2,6 @@ package itest
 
 import (
 	"bytes"
-	"context"
 	"crypto/sha256"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -500,7 +499,7 @@ func runSignVerifyMessage(ht *lntest.HarnessTest, alice *node.HarnessNode) {
 
 	expectedErr := "tag can only be used when the Schnorr signature " +
 		"option is set"
-	ctxt := context.Background()
+	ctxt := ht.Context()
 	_, err := alice.RPC.Signer.SignMessage(ctxt, signMsgReq)
 	require.ErrorContains(ht, err, expectedErr)