Bug 1652149 - Run mozwebidlcodegen unit test as part of mozbuild suite r=froydnj

We could make a new task for this, but `mozwebidlcodegen` depends on code in `mozbuild`, and vice-versa, so there doesn't really seem to be any meaningful advantage to that.

Differential Revision: https://phabricator.services.mozilla.com/D83187

dom/bindings/mozwebidlcodegen/test/python.ini

@@ -1,4 +1,5 @@
 [DEFAULT]
 skip-if = python == 2
+subsuite = mozbuild
 
 [test_mozwebidlcodegen.py]

taskcluster/ci/source-test/python.yml

@@ -368,6 +368,7 @@ mozbuild:
             - '**/moz.configure'
             - 'build/moz.configure/**'
             - 'config/tests/**'
+            - 'dom/bindings/mozwebidlcodegen/**'
             - 'python/mach/**'
             - 'python/mozboot/**'
             - 'python/mozbuild/**'