Add an "assert mode" to `-X showrefcount` that fails when references leak

The -X showrefcount option currently prints the number of references at exit.
Once we get the number to 0, we should test that it stays that way. It would be nice to add a mode, say -X showrefcount=assert0 or -X checkrefcount, that exits with nonzero if refs/blocks remain. Then it can be used in subprocess tests.

cc @vstinner

先追蹤 -X showrefcount 選項的處理方式，以及子程序測試如何呼叫直譯器選項。新增一個 assert 模式：當仍有參照或區塊存在時回報失敗，並以非零結束碼結束，然後在子程序測試中涵蓋此模式；當參照和區塊數為零時該模式通過，而仍有洩漏時失敗，即表示完成。