This commit modifies test_interfaces.js to also test the exposed interfaces
in the WebAssembly namespace. We have conditional features that we'd like
to have confidence that we're not accidentally exposing to web content.
Currently there are:
* WebAssembly exceptions, enabled only in nightly with a default-off pref
* mozIntGemm accelerator function, available only in system or addon principals
Depends on D120731
Differential Revision: https://phabricator.services.mozilla.com/D120732
