[lldb][Docs] Link to testing page from debugging page

The testing page already has some page about debugging failures.

I'm not linking directly to that section because:
* The earlier sections about running single tests and such
  are just as useful for debugging in general.
* The new theme has a nice sidebar on the right that makes
  it really easy to find what you want once on the page.
* We'll probably add more content to the testing page later.