CMake: add manual BUILD_TESTING option

[rouault]

1f8573b removed include(CTest) but that module was useful to define the BUILD_TESTING option. Manually define it

[thbeu]

Hm, not sure actually since set by:

if(BASH_EXECUTABLE)
  set(BUILD_TESTING ON CACHE BOOL "")
else()
  message(STATUS "WARNING: bash not available so disabling testing")
endif()

[rouault]

good point. I missed we manually defined it. Further fix in #164