Add unit tests to create and verify base url permutations.

[jamwil]

Unit tests to verify that a site::Site is successfully built and contains valid permalinks using the new base url logic in src/cmd/serve.rs introduced in #2311.

Closes #2448.

I am marking this as a draft for now. This tests the create_new_site function and verifies the urls and paths, but I'd like to add further tests of the actual live hyper endpoint.

Note that the tests will fail until this rebased on #2482 as that PR makes the trailing slash behaviour consistent with prior before we added the construct_base_url function.

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