test: the file-name test's later backup is later in UTC too

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/backup/archive_test.go

@@ -172,7 +172,11 @@ func TestAFailedWriteFileLeavesNothingBehind(t *testing.T) {
 
 func TestFileNamesSortByTimeAndAreRecognised(t *testing.T) {
 	earlier := FileName(time.Date(2026, 1, 9, 23, 0, 0, 0, time.UTC))
-	later := FileName(time.Date(2026, 1, 10, 1, 0, 0, 0, time.FixedZone("x", 5*3600)))
+	// 01:00 at UTC-5 is 06:00 UTC: the name is in UTC, not the clock's zone.
+	later := FileName(time.Date(2026, 1, 10, 1, 0, 0, 0, time.FixedZone("x", -5*3600)))
+	if later != "minstrel-20260110T060000Z.tar" {
+		t.Errorf("name = %s, want it in UTC", later)
+	}
 	if earlier >= later {
 		t.Errorf("%s does not sort before %s", earlier, later)
 	}