Revert "Move some test related code around."

This reverts commit d92dd482ae06a0df8934b32f4ecad9bbf4254894.
Revert "Tests: Build alt-test-generator tests in a separate binary dir."

This reverts commit 727f4badde1a74652b20ea775277bf968e72b8d1.
Revert "Move the invocation of the CMAKE_SETUP_TESTING macro."

This reverts commit 4746a1d8257e2663845ec952c0faf9e6689d29ae.
Merge topic 'alt-test-generator' into next

d69ff942 Revert "Move the invocation of the CMAKE_SETUP_TESTING macro."
d0835f87 Revert "Tests: Build alt-test-generator tests in a separate binary dir."
c02b4f00 Revert "Move some test related code around."