Remove `multi_thread` flavor from tokio test

Nobody has claimed this yet.

# 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 https://github.com/influxdata/datafusion-udf-wasm/issues/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.

Start with the `test_add_one` Tokio test and trace how non-exact UDF signatures cause `return_type` to be computed synchronously. Review the remaining workaround and the related upstream DataFusion discussion, including issue #31. Done means the test no longer uses `flavor = multi_thread` while preserving its behavior.

Written by the indexing model from the issue text.