Data tests can cause "user query limit" issue if queries queue for over 600 seconds.

[DylanBaker]

With the recent async changes, we now run all data tests at the same time.

This is from the Looker docs:

Per-user query limit and timeout: To prevent any single user from filling up the Looker query queue, each user has a maximum number of allowed concurrent queries and a corresponding queue timeout. By default, each user can run a maximum of 15 concurrent queries, and the timeout for queries queued due to this limit is 600 seconds.

Some organisaitons have a large number of tests, many of which take a while to run. As a result of this, some organisations are getting errors related to the user query limit when the data test queries take a long time.

I believe the solution is to bound the async tasks with a Semaphore. It probably makes sense to make that number 15, though we could also make it configurable.