Re-introduced test/data directory
Re-introduced test/data directory

It was removed by accident in 171e9aae.
In e0fa4cd7, the file `test/data/.gitignore` was restored into `./.gitignore`
while that file effectively makes sure that git stores the `test/data` folder.

I tried to check whether `./.gitignore` was present before, but I couldn't find
one, so I assume this commit sets everything right.

matlab2tikz/matlab2tikz,matlab2tikz/matlab2tikz,RPeschke/matlab2tikz,egeerardyn/matlab2tikz,LJeub/matlab2tikz,okomarov/matlab2tikz,egeerardyn/matlab2tikz,miscco/matlab2tikz,nidrosianDeath/matlab2tikz,PeterPablo/matlab2tikz,okomarov/matlab2tikz