Put tests in a subdir.
Revert "Tests: Don't expect the Tests dir to be called 'Tests'."

This reverts commit cfb566a620939b9d260f58347651de0a108b7237.
Move the invocation of the CMAKE_SETUP_TESTING macro.

Ensure that the CMAKE_TEST_GENERATOR is populated early.
Merge topic 'alt-test-generator' into next

cba43d92 Move the invocation of the CMAKE_SETUP_TESTING macro.
29a9047b Revert "Tests: Don't expect the Tests dir to be called 'Tests'."
3c47cb46 Put tests in a subdir.