Clean up CTest usage.
- Remove use of backwards-compatibility Dart include; we are not using
such dashboards.
- Move CTest include to the top of the main CMakeLists.txt. This clears
a lot of confusion about how BUILD_TESTING works, as it is declared by
CTest. Previously it worked because include(Dart) also declared it.
- The memory check suppression file needs a different treatment. That
also seems to have worked only because CTest was included twice.
Change-Id: I418997bfe01a0f3d90f40558a4dbba10f4744af7