text: Title: Add benchmark test for open_datatree
  Body: ### What is your issue?
  
  To prevent regressions in the future, we should add a benchmark test for opening a deeply nested data tree.
  
  This is to follow up on #9014 @aladinor's performance improvements.
  
  
  You can see here how we benchmark opening and loading a single netCDF file
  
  https://github.com/pydata/xarray/blob/447e5a3d16764a880387d33d0b5938393e167817/asv_bench/benchmarks/dataset_io.py#L125
  
  
  