Fix header_self_contained test and including ArborX_Config.hpp

Arndt, Danielrequested to merge
github/fork/masterleinad/fix_selfcontained into master

Created by: masterleinad

I was seeing that we were not taking one of the CUDA-aware MPI paths even though ARBORX_USE_CUDA_AWARE_MPI was defined. This pull request makes sure that every header actually sees ArborX_Config.hpp. I also noticed that the headers_self_contained test was not working at all. Since we defined ArborX_HEADERS after including the test directory. I added the corresponding tests to the CTest run such that we actually see that they are running. Of course, compiling is sufficient but the overhead appears to be really small.

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