Use ENABLE_TESTING to decide whether to build tests

The BUILD_TESTING variable is a CMake variable that defaults to ON, we
should use one variable consistently to decide whether to enable and
build testing.

Change-Id: If4a331e8e8f908c64845a6b864959903a0b03297
Merge branch 'build-testing'

Change-Id: I0bb7835bd2ddca56986606b68fb61a2ac30fc8ff