Auto merge of #11552 - izgzhen:add-testing-fix-filemanager, r=Manishearth

Improve implementation and add testing regarding file manager thread

First there is a more completed unit test. And in the test running, I found a runtime error `Serde(Custom("bincode does not support Deserializer::deserialize))` when reading response from file manage thread. I analyzed a bit and found that it is probably caused by the `Uuid` field. I temporarily work around it by making the `Id` essentially a string wrapped inside `SelectedFileId`.

Related to PR #11131.

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