Unit testing for reader and writer

[EronWright]

Problem description
The existing tests do not provide adequate coverage because they're unable to simulate a wide variety of edge cases. For example, the writer has logic for write failures and async writes that isn't exercised.

Suggestions for an improvement
Develop a new set of tests that use a mock Pravega client and the Flink-provided operator test harness. See io.pravega.connectors.flink.EventTimeOrderingOperatorTest for an example.

Consider moving those tests that rely on an embedded Pravega server to intTest.

[EronWright]

All readers and writers have unit tests now, except for FlinkPravegaInputFormat which is covered by #78