tests: fix NULL references to be acceptable by Clang

tests: fix NULL references to be acceptable by Clang

 - On some platforms NULL evaluates to nullptr, which will require a
   cast to the right type to be able to compile.
      error: reinterpret_cast from 'nullptr_t' to 'ContextWQ *'

 - It is a delicate change since otherwise GCC will start complaining
   about the reverse:
      error: invalid static_cast from type 'const long int' to type 'ContextWQ*'

 By casting right at the instance of NULL both compilers are happy.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>