test async processing cornercases

[butonic]

We added tests to cover several bugs where file metadata or parent treesize might get corrupted when postprocessing errors occur in specific order.
For now, the added test cases test the current behavior but contain comments and FIXMEs for the expected behavior.

The PR adds eight new test cases:

• the first can succeed before the second succeeds - Works!
• the first can succeed after the second succeeds - Works!
• the first can succeed before the second fails - Works!
• the first can succeed after the second fails - the file should still be in "processing" after second upload is deleted
• the first can fail before the second succeeds - list of revisions should be 0
• the first can fail after the second succeeds - list of revisions should be 0
• the first can fail before the second fails - file should be deleted, size should be correctly reverted
• the first can fail after the second fails - file should be deleted, size should be correctly reverted

I'd like to get this in so it does not get lost in an spreadsheet on my desktop.

@aduffeck And I are entertaining the idea of creating the note on the initiateFile call and keep track of all ongoing uploads in the node metadata. It will allow resolving the correct size diff and processing status. Will create an issue to track that.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[aduffeck]

lgtm