Write some testing helper functions

[Mallory Adams]

• Name the class \Dxw\Iguana\Testing
• Provide a mock for Value/Get - $this->getGetValue([...]); (no longer needed thanks to #22)
• Provide a mock for Value/Post - $this->getPostValue([...]); (no longer needed thanks to #22)
• Provide $this->assertIsRegisterable(...); which just does $this->assertInstanceOf(\Dxw\Iguana\Registerable::class, ...);