Add test coverage (for advancement to Phase 3)

[linclark]

In order to advance wasi-filesystem to Phase 3, we'll need to add test coverage

As discussed in https://github.com/WebAssembly/WASI/issues/384, the Wasmtime team has offered to contribute tests to seed the repo with the fs tests that Wasmtime has been using.

[linclark]

[bookkeeping] Added to the Moving wasi-filesystem to Phase 3 project.