allow ctest to run from the top level build

[cwpearson]

No description provided.

[dssgabriel]

Didn't realize my changes would break this! Feel free to remove the comments if this should be obvious.

Also, does this change allow us to remove the enable_testing() calls that are in the CMake files of the test directories?

[janciesko]

This does not work for me. Should this work?

$pwd
/ascldap/users/jciesko/RAndD/kokkos-comm
$ctest -j 3
Test project /ascldap/users/jciesko/RAndD/kokkos-comm
No tests were found!!!

How do you all feel about set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)?

[cwpearson]

The following works for me and seems to work in our CI

cmake -S ... -B ... -DKokkosComm_ENABLE_TESTS=ON -DKokkos_ROOT=...
cmake --build ...
ctest --test-dir ...