C++ client: clean up some tests

[kosak]

This PR is just doing some mild cleanup:

1. Remove dead code
2. In validation_test remove a call to a helper method. Formerly we had the test entry point calling TestSelects which called TestSelectsHelper. Now we have the test entry point doing some work, then calling TestSelectsHelper.
3. Likewise for TestWheres and TestWheresHelper.
4. In validation_test we now use the printing routines provided by the fmt library (which know how to print a vector) rather than explicitly making our own comma separated list