Test that getrandom works correctly in a (service) worker in web environments.

[briansmith]

Right now the testing for browser environments is "whatever wasm-bindgen-test does." There are several different contexts in which WebAssembly code can be run, and it would be good to be able to test that getrandom is working in all those contexts (extensions, workers), not just in the default execution environmet in a web page. I don't know how to do this testing though. cc @oliverdunk and see briansmith/ring#1508 for background.