PipelineOptions can pick up definitions from unrelated tests

[damccorm]

Since PipelineOptions uses \_\_subclasses\_\_ to look for all definitions, when used in tests it can sometimes pick up sub-classes that were created in previously executed tests.

See BEAM-9975 for more details.

Imported from Jira BEAM-10006. Original Jira may contain additional context.
Reported by: bhulette.