Fix occasional test failure on writing file

[chipsenkbeil]

Seems like the data to be written to the file does not yet exist in the file.

1. Create a temporary file
2. Request writing content to file
3. Receive response with new sig
4. Read file to compare to expected result

File appears to be empty, so guess is that given that the server handles this asynchronously, maybe the file hadn't been written before the response was sent?

Shouldn't be the case as we're waiting for the write to complete first and using write_all of tokio with an await.