Add crappy trick to allow coverage file generation in parallel

tests/mpi_test_main.cpp

@@ -4,11 +4,19 @@
 #include <Kokkos_Core.hpp>
 #include <Messenger.hpp>
 
+#include <cstdlib>
+
 int main( int argc, char* argv[] )
 {
+  setenv("GCOV_PREFIX", "/dev/null", 1);
+
   Messenger::create(argc,  argv);
   Kokkos::initialize({4,-1,-1,true});
 
+  if (commRank() == 0) {
+    unsetenv("GCOV_PREFIX");
+  }
+
   int result = Catch::Session().run( argc, argv );
 
   Kokkos::finalize();