Document EnableTestAssistant

[rahulporuri]

enable.testing.EnableTestAssistant is valuable for developers using enable who want to test their code. We should add some documentation around the test assistant by extracting information from the enable testsuite.

[aaronayres35]

Note there also exists a KivaTestAssistant used internally. It is the base class for EnableTestAssistant and is probably worth including in the documentation as well