WIP: Fix CMakeLists.txt of tests for Conan 2.0

Still crashes with weird linker errors regarding boost::test (??)

usr/bin/ld: /home/joscha/.conan2/p/b/boostec508655df7a4/p/lib/libboost_test_exec_monitor.a(test_main.o): in function `test_main_caller::operator()()':
/home/joscha/.conan2/p/boostada19f4305785/s/src/./boost/test/impl/test_main.ipp:34: undefined reference to `test_main(int, char**)'
/usr/bin/ld: /home/joscha/.conan2/p/b/boostec508655df7a4/p/lib/libboost_test_exec_monitor.a(test_main.o): in function `test_main_caller::operator()()':
/home/joscha/.conan2/p/boostada19f4305785/s/src/./boost/test/impl/test_main.ipp:34: undefined reference to `test_main(int, char**)'
collect2: error: ld returned 1 exit status
gmake[3]: *** [test/CMakeFiles/test_data_type.dir/build.make:223: test/test_data_type] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:347: test/CMakeFiles/test_data_type.dir/all] Error 2
gmake[2]: *** Waiting for unfinished jobs....
collect2: error: ld returned 1 exit status
gmake[3]: *** [test/CMakeFiles/test_configuration.dir/build.make:223: test/test_configuration] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:375: test/CMakeFiles/test_configuration.dir/all] Error 2
/usr/bin/ld: /home/joscha/.conan2/p/b/boostec508655df7a4/p/lib/libboost_test_exec_monitor.a(test_main.o): in function `test_main_caller::operator()()':
/home/joscha/.conan2/p/boostada19f4305785/s/src/./boost/test/impl/test_main.ipp:34: undefined reference to `test_main(int, char**)'
/usr/bin/ld: /home/joscha/.conan2/p/b/boostec508655df7a4/p/lib/libboost_test_exec_monitor.a(test_main.o): in function `test_main_caller::operator()()':
/home/joscha/.conan2/p/boostada19f4305785/s/src/./boost/test/impl/test_main.ipp:34: undefined reference to `test_main(int, char**)'
collect2: error: ld returned 1 exit status
gmake[3]: *** [test/CMakeFiles/test_entity.dir/build.make:223: test/test_entity] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:431: test/CMakeFiles/test_entity.dir/all] Error 2
collect2: error: ld returned 1 exit status
gmake[3]: *** [test/CMakeFiles/test_connection.dir/build.make:223: test/test_connection] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:403: test/CMakeFiles/test_connection.dir/all] Error 2
gmake[1]: *** [Makefile:146: all] Error 2
gmake[1]: Leaving directory '/home/joscha/caosdb/src/caosdb-cpplib/build/Debug'