multiple YaspGrid tests use same temporary file

From test-yaspgrid.hh:

   Dune::BackupRestoreFacility<Grid>::backup(*grid, "backup");

   // avoid that processes that having nothing to backup try to restore
   // a grid that has not been backuped yet.
   grid->comm().barrier();
   Grid* restored = Dune::BackupRestoreFacility<Grid>::restore("backup");

The same file "backup" is used by all YaspGrid backup/restore tests. This is a race condition that can result in tests randomly failing. For example, I just got

50/67 Test #52: test-yaspgrid-backuprestore-tensor ....................***Failed    0.21 sec
 using tensorproduct coordinate container!

Dune::Exception [loadbalance:../dune/grid/yaspgrid/partitioning.hh:128]: Your processor number doesn't match your partitioning information

and

67/67 Test #53: test-yaspgrid-backuprestore-tensor-mpi-2 ..............***Timeout 666.01 sec
 using tensorproduct coordinate container!

 using tensorproduct coordinate container!

Dune::GridError [YaspGrid:../dune/grid/yaspgrid.hh:1004]: Setup of a tensorproduct grid requires monotonous sequences of coordinates.

on one run, but the failure did not happen again during a second run.