testing/regression/* not used by CMake/CTest

[alliepiper]

Naively trying to add these tests caused issues on windows due to CMAKE_OBJECT_PATH_MAX -- the paths to the object files exceed a system-dependent maximum number of characters. We should find a way to add these.

We could shorten the names, but it might be best to just integrate them with the other unit tests in testing/ or testing/<backend> and put bug metadata in comments.

[alliepiper]

Just noticed that some of these aren't meant to be executed and just check that compilation succeeds, e.g:

https://github.com/NVIDIA/thrust/blob/main/testing/regression/gh_911__merge_by_key_wrong_element_type_default_comparator.cu

I can't find an existing build system for these, so I'm not sure which are meant to be run (if any). @brycelelbach do you recall if these are all intended to be compile only tests?

[brycelelbach]

Some of them are, some are meant to be run. It was an idea that never really came to fruition.

[alliepiper]

We should incorporate the ones that run into the existing unit tests and remove this directory, since it's experimental / unused.

[alliepiper]

There's some overlap here with #807 -- once we have proper compile test support we could use that to check the "expected build failure" tests.