Bug 1721686 - Add test for exposed interfaces on WebAssembly namespace. r=smaug

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