Add ValidatesRunner Tests for Custom Windows, WindowCoders that do not define good equals methods

This exercises a good portion of the runner logic which should only compare windows based on their structural values and/or encoded bytes.

Imported from Jira [BEAM-2045](https://issues.apache.org/jira/browse/BEAM-2045). Original Jira may contain additional context.
Reported by: tgroh.

Start by locating the ValidatesRunner tests and the runner logic that compares custom Windows and WindowCoders. Read how structural values and encoded bytes are currently compared, then add coverage for coders without reliable equals methods. Done means the tests exercise these cases and pass without relying on object equality.

Written by the indexing model from the issue text.