Run tests under WebAssembly in CI

[robertknight]

CI currently builds the library for WebAssembly, but doesn't run tests. cargo test doesn't work directly with a WASM target, but tests can be built and then run using eg. wasmtime:

cargo build --tests -p rten --target wasm32-wasi
wasmtime target/wasm32-wasi/debug/deps/rten-{build_hash}.wasm

From a quick test just now, all the tests can run except for test_rnn_pytorch which tries to read a local file, which is disallowed. I believe there is a way to read the file, but we also want to make sure that the library will work in a browser environment where those APIs are not available.