add test for 'ipfs files mkdir -p'

We want to stress mkdir with and without the `-p` option. And also want to make sure that both get called on directories that already exist (to assert that nothing happens, and their contents don't change).

Also want to make sure we run `mkdir -p` on paths that already partially exist (i.e. `mkdir -p a/b/c/d`when a/b already exists)