Add declared const Buffer spec

spec/fixtures/module/declare-buffer.js

@@ -0,0 +1,2 @@
+const Buffer = 'declared Buffer'
+module.exports = Buffer

spec/modules-spec.js

@@ -69,6 +69,12 @@ describe('third-party module', function () {
         assert.strictEqual(require('./fixtures/module/declare-global'), 'declared global')
       })
     })
+
+    describe('Buffer', function () {
+      it('can be declared in a module', function () {
+        assert.strictEqual(require('./fixtures/module/declare-buffer'), 'declared Buffer')
+      })
+    })
   })
 })