Bug 1579156 - Fix binding tests after bug 1573566. r=chmanchester

The build system has no clue that there is something to build in
dom/bindings/test. It's currently all handled via make rules generated
by the backend, but ideally, this would all be handled by the frontend
emitting appropriate GeneratedFiles and Sources objects.

In the meanwhile, we just force the make backend to recurse through
dom/bindings/test.

Differential Revision: https://phabricator.services.mozilla.com/D45124

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -1827,3 +1827,9 @@ class RecursiveMakeBackend(CommonBackend):
         webidls_mk = mozpath.join(bindings_dir, 'webidlsrcs.mk')
         with self._write_file(webidls_mk) as fh:
             mk.dump(fh, removal_guard=False)
+
+        # Add the test directory to the compile graph.
+        if self.environment.substs.get('ENABLE_TESTS'):
+            self._compile_graph[mozpath.join(
+                mozpath.relpath(bindings_dir, self.environment.topobjdir),
+                'test', 'target-objects')]