Bug 1763236 - Add class documentation for fundamental building block modules of our test infrastructure, r=sclements,mossop

This is not perfect, but I figured getting most of this stood up will make it easier for
other folks to improve, and is a lot better than expecting people to figure most of this
out themselves. Some known issues:

- alphabetical method ordering isn't necessarily what we want here. I haven't invested
  heavily in improving this. I think this can be a follow-up.
- no markdown or HTML support in sphinx-js so the formatting is... not good sometimes.
- types somehow get a bonus `()` chucked after them (so are listed as e.g. `String()`
  rather than just `String`). Unclear what is causing this; it's happening in other
  extant documentation, too, so I expect is a sphinx-js issue.

Differential Revision: https://phabricator.services.mozilla.com/D161650

docs/conf.py

@@ -56,6 +56,11 @@ js_source_path = [
     "../browser/components/uitour",
     "../browser/components/urlbar",
     "../remote/marionette",
+    "../testing/mochitest/BrowserTestUtils",
+    "../testing/mochitest/tests/SimpleTest/SimpleTest.js",
+    "../testing/mochitest/tests/SimpleTest/EventUtils.js",
+    "../testing/modules/Assert.jsm",
+    "../testing/modules/TestUtils.jsm",
     "../toolkit/actors",
     "../toolkit/components/extensions",
     "../toolkit/components/extensions/parent",