Tests fail for target wasm32-wasi

Running `cargo build --target wasm32-wasi` succeeds, but `cargo wasi test` fails:

- The `pathbuf` fails since it relies on `env::temp_dir()`, which currently panics when running in wasi (see: https://github.com/WebAssembly/WASI/issues/306 )
- The `prop_oob` and `testable_unit_panic` tests fails: under wasi, this test panics. When decorating with `#[should_panic]`, they fails under normal (non-wasi) tests

Start by running `cargo wasi test` and compare the failures in `pathbuf`, `prop_oob`, and `testable_unit_panic` with normal Rust tests. Trace the affected test code and the `env::temp_dir()` behavior under wasm32-wasi; done means the suite passes for both wasm32-wasi and the normal target without breaking panic expectations.

Written by the indexing model from the issue text.