Remove the need for --test-pipeline-options in flink/spark Python tests.

Flink and Spark Python tests are skipped if --test-pipeline-options is unset, but that's just an unfortunate implementation detail of the test framework - we want to skip Flink and Spark tests in the general pytest suite. We should find a better way to know to skip these tests, so that there's less cognitive load in trying to figure out what --test-pipeline-options should be set to.

https://github.com/apache/beam/blob/51541a595b09751dd3dde2c50caf2a968ac01b68/sdks/python/apache_beam/runners/portability/flink_runner_test.py#L81