Improve test_basic_dir test

[dlrobertson]

We could use tempdir to create a directory which we wouldn't have access to, or do something like a lookup in "./tests". We probably should avoid using "./src" and "./tests" which I think wouldn't work in some cases if the tests binary was run manually outside of the root crate directory.

Originally posted by @dlrobertson in #7 (comment)

Also see https://github.com/dlrobertson/capsicum-rs/pull/7/files#r1016042775