Description

In current api/test_sfp.py, it assumes all SFPs are present. However, in real test environment, there could be SFPs which are not present. It should allow sfp.get_presence return false. And it should check sfp.get_presence before doing further test.

Steps to reproduce the issue:

1. Plug out a SFP
2. Run api/test_sfp.py

Describe the results you received:

Test case should pass

Describe the results you expected:

Test case failed.