For #8804 we skipped some of the FileSystem tests (see #8810). These tests need to be re-enabled and fixed. Also fix the [failing `createWriteStream()` test](https://github.com/microsoft/vscode-python/issues/8804#issuecomment-559135265). Also https://github.com/microsoft/vscode-python/issues/8804#issuecomment-559211063
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