Merge bitcoin/bitcoin#31544: cmake: Remove unused BUILD_TESTING variable from "dev-mode" preset

e196190 cmake: Remove unused `BUILD_TESTING` variable from "dev-mode" preset (Hennadii Stepanov)

Pull request description:

  On the master branch @ bb57017:
  ```
  $ 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`](https://cmake.org/cmake/help/latest/module/CTest.html) module, which we do not include in the project.

ACKs for top commit:
  TheCharlatan:
    ACK e196190

Tree-SHA512: 8110a0f5bdcdd0844ce7dd75160a61d8b3aff95e12da1ec4d55c56c82da41145736da0fad072adeb97551c99e46683a3493435c3bac7d8e4e62ea6086f60fb7a

Author: fanquake

CMakePresets.json

@@ -71,7 +71,6 @@
         "BUILD_GUI_TESTS": "ON",
         "BUILD_KERNEL_LIB": "ON",
         "BUILD_SHARED_LIBS": "ON",
-        "BUILD_TESTING": "ON",
         "BUILD_TESTS": "ON",
         "BUILD_TX": "ON",
         "BUILD_UTIL": "ON",