Add enough test coverage with arbitrary configs test

We started adding some [arbitrary config](https://github.com/citusdata/citus/tree/master/src/test/regress/citus_tests/arbitrary_configs#arbitrary-configs) tests, and it seems there are still a lot of room to improve, and we need some help.

We'd appreciate if you pick some items from the list and add tests within the next few weeks. I want us to add tests for the following areas before the next release. If you see anything missed, please add to the list.

I suggested some names for each test considering the recent work you had.

- [x] Planner
- [x] The queries that touch different planners
- [x] Repartitioning
- [x] Intermediate Results
- [x] Local-dist table joins
- [x] CH Benchmark
- [x] Executor
- [x] Dropped Columns
- [x] Prepared Statements
- [x] TRUNCATE (@gledis69 )
- [ ] Columnar [ ] (@onurctirtir )
- [ ] Alter table UDFs / Undistribute table [ ] (@halilozanakgul )
- [ ] Function evaluation [ ] (@onderkalaci )
- [x] Sequences [ ] (@agedemenli )
- [ ] Function call delegation [ ] (@tejeswarm )
- [ ] Object propagation [ ] (@thanodnl )
- [ ] Insert .. SELECT commands (@onderkalaci )
- [ ] Partitioning UDFs via 10.2 [ ] (@velioglu )
- [x] DDL commands [ ] (@hanefi and @agedemenli )
- [ ] Rebalancer / Drain Node [ ] (@JelteF )