Declare paid event as example data, call from test

Declare paid event as example data, call from test

I'm looking to create some set up helper classes in
the Civi\Test space, to also be available to extensions
& to standardise some of the set up routines.

I figured the best place to put the ExampleData
was in the structure created for that.

The ExampleData structure is lightly used by
the WorkflowMessage::render() function for personal
entities (ie Contact,Contribution) but it
is not used (& probably won't be) for
configuration entities. However, the examples
are intended to be multi-purpose so I think it is the right location.

Note the trait that builds on it will have to have
a lot more logic to build price sets etc than the exampleData
space really supports