Add integration tests to kserve server rocks

[ca-scribner]

Context

The servers in this repo have simple integration tests described in their respective readme.md files - we should automate these and use them for CI/manual testing.

I tried looking into this a bit and what I got hung up on was how we needed to docker run the image, then curl at it. I wasn't sure how to manage this with tox. I looked into using tox-docker which does almost this, but it doesn't support passing args to the container. There were possible workarounds, but I didn't dig into it much

What needs to get done

1. automate the integration tests from readme.md for all server images

Definition of Done

1. server rocks have automated integration tests

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5429.

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[ca-scribner]

@orfeas-k also raised a good point that we could probably write the test purely in pytest. that might make it easier