Make sure we unit test bounded sessions

A few customers have been using Window.into(Sessions...) and of course quickly realize they are exposed to unbounded sessions.

We should have unit tests to confirm various combinations of AfterPane.elementCountAtLeast and AfterProcessingTime... correctly force sessions to be broken apart.

We should also check this all works with repeated messages with the same timestamp (since they will create the exact same session window and can thus see trigger state from previous sessions).

At some point we may may flow into reworking bounded sessions to be done directly rather than via Sessions plus triggers.

Imported from Jira [BEAM-279](https://issues.apache.org/jira/browse/BEAM-279). Original Jira may contain additional context.
Reported by: mshields822.

Start by locating the Apache Beam unit tests for Window.into(Sessions...), AfterPane.elementCountAtLeast, and AfterProcessingTime. Add coverage for combinations that should break bounded sessions apart, including repeated messages with the same timestamp, and run the relevant test target to verify those cases.

Written by the indexing model from the issue text.