Add matrix-free tests

[Rombur]

We need to add tests that use laplace_matrix_free

[dalg24]

1. make eigensolvers work for matrix free (blocked by Integrate Wayne's eigensolver #115)
2. implement matrix-free version of the mesh evaluator that does not derive from the regular one
3. provide a new version for all mesh evaluators in the unit tests :( (blocked by 2.)

[dalg24]

1. make eigensolvers work for matrix free (blocked by Integrate Wayne's eigensolver #115)
2. implement matrix-free version of the mesh evaluator that does not derive from the regular one
3. provide a new version for all mesh evaluators in the unit tests :( (blocked by 2.)

First point has been addressed in #127 and #136. Second one is partially covered by #137.