docs(testing): use marked Jade filter (#2444)

When the testing page is rendered the following warning is generated:
```
Transformers.markdown is deprecated, you must replace the :markdown
jade filter, with :marked and install jstransformer-marked before you
update to jade@2.0.0.
```

Author: chalin

public/docs/ts/latest/guide/testing.jade

@@ -518,7 +518,7 @@ a(href="#top").to-top Back to top
   ### The tests
   Jasmine runs this `beforeEach` before each test of which there are two
 +makeExample('testing/ts/app/banner.component.spec.ts', 'tests', 'app/banner.component.spec.ts (tests)')(format='.')
-:markdown
+:marked
   These tests ask the `DebugElement` for the native HTML element to satisfy their expectations.
 
 #detect-changes