plugin should be able to close

[manyuanrong]

Currently openPlugin will create a resource and add it to resourse_table. But now it cannot be closed, resulting in the failure of the test case

Error: Test case is leaking resources.
Before: {
  "0": "stdin",
  "1": "stdout",
  "2": "stderr"
}
After: {
  "0": "stdin",
  "1": "stdout",
  "2": "stderr",
  "6": "plugin"
}
    at Object.assert ($deno$/util.ts:25:11)
    at resourceSanitizer ($deno$/testing.ts:62:5)
    at async TestApi.[Symbol.asyncIterator] ($deno$/testing.ts:222:11)
    at async runTests ($deno$/testing.ts:398:20)