allow bolero tests to run on wasm32-unknown-unknown

[Ekleog]

This PR allows crate developers to run bolero::check!() tests on wasm32-unknown-unknown.

This is not fuzzing yet, but at least basic proptesting works there, with this PR :)

Considering the infrastructure required for testing (all the wasm-bindgen stuff), I'm not submitting a new test here; but I checked on the repo with my use case that a test that takes an u8 and matches for != 4 did fail as expected.

Basing the test on target_os should also (hopefully) make bolero work for embedded development proptesting.