Improved unit tests for open_maybe_zipped function.

[levahim]

This PR is to address requests formulated in the comments to PR #13984. Namely:

1. As requested by @mik-laj the airflow.utils.file unit tests moved out of test_dag_processing.py into its own module test_file.py.
2. As requested by @ashb the open_maybe_zipped function unit test, instead of relying on mocks, now actually reads a test file contents, zipped and plain, and verifies that the returned file content is string in both cases.

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[levahim]

The pylint and k8s test errors appear to be completely unrelated to what's in this PR. Seems to be something that came from master.

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[mik-laj]

CI are sad. Can you do a rebase?

[github-actions]

The Workflow run is cancelling this PR. Building images for the PR has failed. Follow the the workflow link to check the reason.

[levahim]

The CI pipelines cancelled with error: "The self-hosted runner: Airflow Runner 60 lost communication with the server. Verify the machine is running and has a healthy network connection. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error." I don't think it's under my control. Can you restart the build? (Or let me know if I can restart it e.g. with special comments.)