restore example targets and test script functionality

[StillGreen-san]

this restores the example targets and test script to a working state

in #25 the creation of the example targets was changed, but that broke some things

• the example now got build into a sub directory but test.sh was not updated to account for that
• git2cpp was no longer linked into the target, failing to compile
• all examples got added as a single executable, which does not work since they all have there own main, preventing linking

I choose to update test.sh to look inside the examples sub directory. alternatively test.sh would need to be copied into the sub dir or the example target creation moved back into the main CMakeLists.txt

tested with BUNDLE_LIBGIT2=OFF