Aggregator Verifier and Integrate to E2E Test Framework

[kennknowles]

Create a matcher to verify the value of Aggregator, which is AggregatorValues, by given expected values.

For example:

MyFnWithAggregator fn = new MyFnWithAggregator();
Pipeline p = ... (setup the pipeline, use fn) ...
p.run();
assertThat(pResult,
isAggregatorValue(values, fn.aggregator));

It also should be integrated in E2E pipeline test and used by setting onSuccessMatcher.

Imported from Jira BEAM-546. Original Jira may contain additional context.
Reported by: markflyhigh.