cmake: Remove unused `BUILD_TESTING` variable from "dev-mode" preset
On the master branch @ bb57017b:
$ cmake -B build --preset dev-mode -DWITH_MULTIPROCESS=OFF
<snip>
-- Configuring done (12.0s)
-- Generating done (0.1s)
CMake Warning:
Manually-specified variables were not used by the project:
BUILD_TESTING
-- Build files have been written to: /home/hebasto/git/bitcoin/build
This PR resolves the issue.
The removed BUILD_TESTING variable is a part of the CTest module, which we do not include in the project.