Grids for the test-suite

Much of Ferrite's code is only tested for grids from the `generate_grid` function, but @koehlerson and I discussed that it would be valuable to have some grids available for testing that unstructured, stemming from e.g. Gmsh and Abaqus.

We recently found some code, not yet in Ferrite, that worked well on grids from `generate_grid`, but not for unstructured grids.

One suggestion would be to have a list of different types of relatively small grids for use with testing, both with the same cell type and mixed celltypes. One possibility could be to define something like
```julia
const test_grids = Dict(
"GenTriangle" => generate_grid(Triangle, (3,3)),
"GmshTriangle" => togrid(joinpath(@__DIR__, "triangle.msh")),
"AbaTriangle" => get_ferrite_grid(joinpath(@__DIR__, "triangle.inp")),
"GmshTriQuad" => ...
"AbaTriQuad" => ...
)
```
just to make sure to cover some special cases during testing.
This would also serve as "reverse-ci" (cf. #797) for the mesh parsing.

Thoughts?