Add BUILD_TESTING to standard CMake arguments (#30374)

CTest determines whether to enable tests using the BUILD_TESTING variable.
This should be used by projects to conditionally enable the compilation of tests.
Spack knowns which packages have to run tests and can thus automatically define this variable.

lib/spack/spack/build_systems/cmake.py

@@ -176,6 +176,7 @@ def _std_args(pkg):
             '-G', generator,
             define('CMAKE_INSTALL_PREFIX', convert_to_posix_path(pkg.prefix)),
             define('CMAKE_BUILD_TYPE', build_type),
+            define('BUILD_TESTING', pkg.run_tests),
         ]
 
         # CMAKE_INTERPROCEDURAL_OPTIMIZATION only exists for CMake >= 3.9
@@ -361,6 +362,7 @@ def cmake_args(self):
 
             * CMAKE_INSTALL_PREFIX
             * CMAKE_BUILD_TYPE
+            * BUILD_TESTING
 
         which will be set automatically.