[!400] Fix tests of nested function spaces in testordering

Merge branch 'bugfix/fix-bulk-testordering-bug' into 'master'

ref:pdelab/dune-pdelab Right now, testordering just calls map_lfs_indices() even on nested function spaces. That won't work, because the inner ordering nodes expect the leaf nodes to have already processed the DOFs.

Fixed by using the visitor from the lfs index cache that correcly handles the nested calls.

See merge request pdelab/dune-pdelab!400

(cherry picked from commit d6cbf6bc)

2846c0ce Fix tests of nested function spaces in testordering