Remove multi_thread flavor from tokio test #169
Description
What
One of the tests we have, test_add_one; relies on the multi-threaded tokio test 'flavor' in order to run. This is because when we register UDFs that do not have an exact function signature, we must compute them every time when calling return_type. Which is not async; we compute the UDF return_type asynchronously and store it for later if it is exact. test_add_one does not register a function with an exact signature, so we cannot compute the return type asynchronously.
To get around return_type being sync, we had/have a hack in place that was mostly fixed in #31. This hack is still around though, due to effectively needing to block to compute the return type if the function signature isn't exact (mentioned above).
What we need is to completely remove the use of flavor = multi_thread from the tokio test.
How
This will likely involve making changes upstream to DataFusion, this needs to be discussed further.