Rename cpp/test to cpp/tests.

[bdice]

Renames test directories to tests for alignment with the rest of RAPIDS.

See also: rapidsai/build-planning#140

[vyasr]

/merge

[bdice]

Another one of the strange cuda-gdb errors:

  File "/__w/cuml/cuml/python/cuml/cuml/tests/test_no_cuinit.py", line 77, in test_cuml_import_no_cuinit
    assert output.returncode == 0
AssertionError: assert 127 == 0
 +  where 127 = CompletedProcess(args=['/usr/local/cuda/bin/cuda-gdb', '-x', '-', '--args', '/pyenv/versions/3.10.16/bin/python', '-c'...rror while loading shared libraries: libpython3.9.so.1.0: cannot open shared object file: No such file or directory\n').returncode
----------------------------- Captured stdout call -----------------------------
Command output:

*** STDOUT ***

*** STDERR ***
/usr/local/cuda/bin/cuda-gdb-python3.9-tui: error while loading shared libraries: libpython3.9.so.1.0: cannot open shared object file: No such file or directory

[vyasr]

Until we can get that to repro on a given runner I've sort of given up on it for now :(