Turning off autovacuum for the tests is a valid option and I will definitely do this as a workaround. Each test pretty much starts with empty schema and data for it is generated during the run and rolled back at the end. I have a lot of tests and at the moment it is not feasible to modify them.
The real workload for the application is different, but there are some cases, when we import data from remote web service in a transaction do some work with it and then we do a commit. If there is an autovacuum during this process I assume there will be similar problem regarding planner statistics.
The real problem here is that the statistics that are seen from one (uncommited) transaction are affected by analyze statement from another (commited) transaction. I've seen similar behavior in production for other applications using PostgreSQL too (10.x and 11.x) - 100% CPU consumed by an otherwise fine select query. I was puzzled by this behavior back then but now it makes sense.
I assume it is in the documentation, but I am not aware of how stats are handled for uncommitted work. Obviously in the example you provided the table would be empty, but in your real tests do they start out empty? Would it suffice to use temp tables created like the regular ones and analyze after insert to ensure stats are up to date? Or would it make sense to turn off auto-vacuum/analyze during these tests? The workload is not expected to be similar to real world I would think, and so you don't need the vacuum and the stats update perhaps.
