[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