Making sure no zero length arrays are used in tests.

The check_all<...> variadic class template resolves to 0 - length arrays being
declared when check_all<> is instantiated, which is forbidden by 8.3.4 of the
C++11 standard. By introducing the nozero_array constexpr function this is
avoided in combination of the bounds checking that check_all<...> already
implements.
Added two empty tests for testing the "check" template itself.