[bugfix] Fix compile error with NDEBUG

It seems that the `#if ...` was originally introduced to avoid
a compiler warning in case later `assert()` are removed due to
NDEBUG. When changing to `TestSuite` in e7b592a2 this
was not adjusted.

**Surprisingly no one compiled our tests with NDEBUG since June 2017 otherwise this would have triggered a compile error!**

dune/common/test/mpicollectivecommunication.cc

@@ -38,9 +38,7 @@ int main(int argc, char** argv)
     }
 
     // result from above should be size of job
-#ifndef NDEBUG
     double sum = mpi.size();
-#endif
     for(int i=0; i<length; ++i)
     {
       t.check( std::abs( values[i] - sum ) < 1e-8 );