ants: fix build by setting BUILD_TESTING=OFF (#26768)
ants: fix build by setting BUILD_TESTING=OFF (#26768)

Due to Kitware API changes, default ANTs builds were failing, presumably for all versions (https://github.com/ANTsX/ANTs/issues/1236).
This commit defaults BUILD_TESTING to OFF, preventing calls against
these APIs and fixing all versions.
Note that the ANTs test suite was not clean anyway (e.g. ANTs/#842).
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack