Force 4 threads for unit tests

Seems necessary to impose more than 2 threads to activate join() on these simple
examples

tests/test_main.cpp

@@ -5,7 +5,7 @@
 
 int main( int argc, char* argv[] )
 {
-  Kokkos::initialize({-1,-1,-1,true});
+  Kokkos::initialize({4,-1,-1,true});
 
   int result = Catch::Session().run( argc, argv );