Support Verifiers in TestFlinkRunner

[~jasonkuster] suggested that we should support verifiers to better support E2E tests.

See https://github.com/apache/incubator-beam/blob/master/examples/java/src/test/java/org/apache/beam/examples/WordCountIT.java for an example of how they're used and https://github.com/apache/incubator-beam/blob/master/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/testing/TestDataflowRunner.java for how they are implemented in the TestDataflowRunner.

Imported from Jira [BEAM-581](https://issues.apache.org/jira/browse/BEAM-581). Original Jira may contain additional context.
Reported by: aljoscha.

Start by reading examples/java/src/test/java/org/apache/beam/examples/WordCountIT.java to see how verifiers are used, then compare their implementation in runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/testing/TestDataflowRunner.java. Inspect TestFlinkRunner and add equivalent verifier support for end-to-end tests; done means the WordCountIT-style usage works with the Flink test runner.

Written by the indexing model from the issue text.