Move example selector generation into KWExample
Move test invocation creation into KWSpec; stop using "dummy" invocations

Previously, test invocations were only dummy invocations and were never
executed. Instead, -invokeTest would be called on the KWSpec instance
for each example.

This renames -invokeTest to -runExample, and ensures a valid test
invocation is created for teh -runExample method.

Invocation creation has also moved to KWSpec as it requires knowledge of
private KWSpec methods.
Ensure anonymous it nodes in the same suite have unique selectors