[cmake][test] Cleanup build system for tests

UG and IPOpt flags are both registered via dune_register_package_flags(). Hence they need not be supplied if any of

dune_target_enable_all_packages()
dune_enable_all_packages()

is used. The dune_add_test() function, however, calls add_dune_all_flags() which is the predecessor of dune_target_enable_all_packages() and has the same effect. Hence we can drop all conditionals, lists, and loop here.