iox-#280 fix: Add logic to enable building iceoryx_hoofs_testing
iox-#280 fix: Add logic to enable building iceoryx_hoofs_testing

Useful if a parent project does a find_package to Gtest

Signed-off-by: Andre Nguyen <45b2d910513180512ea92d3a0665e636be6f1ce4@gmail.com>

eclipse-iceoryx/iceoryx,eclipse-iceoryx/iceoryx,eclipse-iceoryx/iceoryx,eclipse-iceoryx/iceoryx