[GSoC] Add unit tests for tune API

[helenxie-bit]

What this PR does / why we need it:
This PR adds unit tests and fix some minor bugs of tune API.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[helenxie-bit]

/area gsoc

[helenxie-bit]

Ref: #2339

[andreyvelich]

Thank you for doing this @helenxie-bit!

[andreyvelich]

@helenxie-bit Let's add the unit tests under the katib_client_test.py file since we previously agreed to place unit test files near the actual files.
You can use the tests data for tune API and define the fixture, similar to other SDK APIs.

[helenxie-bit]

@andreyvelich Thank you! I have followed this instruction and updated unit tests for tune API. Please review it when you have time.