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.

invoices/sql_migration_test.go

@@ -1,7 +1,6 @@
 package invoices
 
 import (
-	"context"
 	crand "crypto/rand"
 	"database/sql"
 	"math/rand"
@@ -305,7 +304,7 @@ func TestMigrateSingleInvoiceRapid(t *testing.T) {
 func testMigrateSingleInvoiceRapid(t *rapid.T, store *SQLStore, mpp bool,
 	amp bool) {
 
-	ctxb := context.Background()
+	ctxb := t.Context()
 	invoices := make(map[lntypes.Hash]*Invoice)
 
 	for i := 0; i < 100; i++ {