Fix regression: multiple use of CommonCPPCTest
Fix regression: multiple use of CommonCPPCTest

MGL uses CommonCPPCTest in more than one test sub-directory,
using the functionality introduced in #d081493. Commit #603eeb5
breaks this capability. This patch creates the
${PROJECT_NAME}_run_cpp_tests target only if it does not
already exist.

shurikasa/CMake,shuaibarshad/KAUST-CMake,biddisco/CMake,shuaibarshad/KAUST-CMake,jafyvilla/CMake,jafyvilla/CMake,biddisco/CMake,jafyvilla/CMake,ptoharia/CMake,shurikasa/CMake,ptoharia/CMake,ptoharia/CMake,shurikasa/CMake,biddisco/CMake,shurikasa/CMake,biddisco/CMake,shuaibarshad/KAUST-CMake,jafyvilla/CMake,ptoharia/CMake,shuaibarshad/KAUST-CMake