Fix calling ycm_ep_helper when YCM is consumed via find_package(YCM) and add test

[traversaro]

The ycm_ep_helper used the two files RepositoryInfo.txt.in and gitsafeclone.txt.in, that however were not installed, so since YCM 0.16.7 and until YCM 0.18.0, ycm_ep_helper worked fine (with CMake >= 3.22) only if YCM was bootstrapped or used via FetchContent, not if YCM was found via find_package(YCM).