Configure required googletest when tests are off, which it shouldn't

With ```-DBUILD_TESTING=OFF``` configure fails:
```
CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
Could NOT find GTest (missing: GTEST_LIBRARY GTEST_INCLUDE_DIR
GTEST_MAIN_LIBRARY)
Call Stack (most recent call first):
/usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
/usr/local/share/cmake/Modules/FindGTest.cmake:270 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
Modules/ThirdParty/GoogleTest/itk-module-init.cmake:36 (find_package)
CMake/ITKModuleEnablement.cmake:338 (include)
CMakeLists.txt:542 (include)
```

Version: 5.3rc04
FreeBSD 13.1