Aggregator Verifier and Integrate to E2E Test Framework

Create a matcher to verify the value of [Aggregator](https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Aggregator.java), which is [AggregatorValues](https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/AggregatorValues.java), 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](https://issues.apache.org/jira/browse/BEAM-546). Original Jira may contain additional context.
Reported by: markflyhigh.

Start by reading Aggregator.java and AggregatorValues.java to understand the value being matched, then inspect the existing E2E pipeline test framework and its onSuccessMatcher usage. Add coverage for the expected AggregatorValues and integrate the matcher into an E2E pipeline test so the success matcher verifies the aggregator result.

Written by the indexing model from the issue text.