[CMake] Disable the test magic in the core modules.
We decided to abandon the testing magic provided by dune-common. Until it is abandoned upstream, it will still be in place and spit out an error message, that is hard to understand for users.
This commit sets a flag that disables the testing magic upstream.
BLOCKED: This only works after dune-common/FS1694-allow-disabling-testing-magic-from-downstream-modules is merged into release-2.4.