How many jobs to list in Dataflow tests

[davidcavazos]

Describe the issue

For Dataflow tests, in
dataflow_job_id_from_job_name,
to get the job ID from a job name, we're currently listing only the latest 50 active jobs.

Currently this is hardcoded to 50, which should be enough for most cases. Right now, there aren't any tests using this function so it shouldn't be an issue. At this point, we could safely remove this function.

For more information, see #5692 (comment)

[davidcavazos]

The page size is now configurable, but independently from that, we now iterate over all the pages until the job is found (typically in the first page anyways).

https://github.com/GoogleCloudPlatform/python-docs-samples/pull/6475/files#diff-4effce466abc249623c22915381350e1d317e0b2830821d21e1de7392f8443e4R280-R294