Add tests for container creation

[NoelDeMartin]

I recently found an issue in NSS nodeSolidServer/node-solid-server#1703, and I was looking at this repository to see if that was covered in this test suite, but I think it isn't.

There is a test covering the intermediate containers creation, but it only seems to test it when creating non-container resources.

[edwardsph]

There are a few related requirements that should be tested:

• https://solidproject.org/TR/protocol#server-post-container
• https://solidproject.org/TR/protocol#server-post-container-create-resource
• https://solidproject.org/TR/protocol#server-post-container-create-container
• https://solidproject.org/TR/protocol#server-put-patch-intermediate-containers

Suggestions:

• Test for 2xx/301 responses
• For POST, test with and without slug