Release reporter destination references after finalization

reporter.go

@@ -327,6 +327,8 @@ func (r *Reporter) finalize(outcome Outcome, completion func(*Reporter)) Outcome
 	state.closed = true
 	state.deferred = nil
 	outcome.Presentation = state.presentationLocked()
+	state.output.writer = nil
+	state.diagnostics.writer = nil
 	state.mu.Unlock()
 	return normalizeOutcome(outcome)
 }