Move elixirs that are only relevant for testing to `test` directory

I already suggested this back in #569 when we organized the elixirs in folders based on mesh type, but I would like to give it another thought.
There are some elixirs that have absolutely no practical use other than testing. I know about the two elixirs that I wrote for AMR testing, `tree_2d_dgsem/elixir_advection_amr_refine_twice.jl` and `tree_2d_dgsem/elixir_advection_amr_coarsen_twice.jl`, but I'm assuming there are many more elixirs like this.
There is absolutely no reason why any user would use these elixirs. They are only there to cover the case of refining/coarsening a cell twice within one AMR callback.

Since there are already a lot of elixirs, it would make things easier for users if elixirs like this would be moved to the `test` directory.

Start by inspecting the elixirs named in the issue, tree_2d_dgsem/elixir_advection_amr_refine_twice.jl and tree_2d_dgsem/elixir_advection_amr_coarsen_twice.jl, and compare them with the test directory. Identify other elixirs used only for testing, move them there, and verify that the test suite and user-facing elixir organization remain correct.

Written by the indexing model from the issue text.