cargo test will test examples that have a "main.rs", but in this case, the example requires manual compilation as a wasm module, and doesn't build as a standalone program. Rename it to "demo.rs".
Build example's file
To build demo.wasm use rustc (nightly) for wasm32 target with debug information:
rustc +nightly --target=wasm32-unknown-unknown demo.rs --crate-type=cdylib
Run example
Point path to the built wasmtime_py library location when running python, e.g.
PYTHONPATH=../../target/debug python3 run.py