Improve tests for flow function

[atharvashahane28]

Currently, the tests for the flow endpoint function cover only the most basic functionality. They do not check the pass/error status of flow, the validity of the output, or the properties of the output.

Todo:
Add tests to:

• Check the pass/error status of flow
• Verify the existence of the output
• Verify the properties of the output (attributes, length, contents)