Get headless browser tests running on Safari in CI

[fitzgen]

Should be similar to our existing wasm-pack tests that run in firefox and chrome, but we need to use a macos vm image, and I'm not 100% sure how to do that in azure pipelines yet!

See .azure-pipelines.yml and .ci/wasm-pack-test-headless.yml for more details.

[rylev]

I can take this on! 😄