add kserve nightly badge

[agunapal]

Description

This PR adds the KServe nightly badge to the README

• It also fixes the test logic to clean up port-forwarding
• Passing run: https://github.com/pytorch/serve/actions/runs/7863937837/job/21455109966

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Feature/Issue validation/testing

• Passing run: https://github.com/pytorch/serve/actions/runs/7863937837/job/21455109966

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?