Build VFS of correct dim in errornorm
Add more tests of assembly cache correctness

Effectively this is turning the assembly cache bug reports into tests.
assembly_cache: Ensure copied matrix is assembled
Check cache size as well as values
Merge pull request #305 from firedrakeproject/fix/parallel_mixed_assembly_cache

* origin/fix/parallel_mixed_assembly_cache:
  Check cache size as well as values
  assembly_cache: Ensure copied matrix is assembled
  Add more tests of assembly cache correctness
  Build VFS of correct dim in errornorm