{storage-blob-preview} add test

[Juliehzl]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• [ ] Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• [ ] Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• [ ] My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your PR is merged into master branch, a new PR will be created to update src/index.json automatically.
The precondition is to put your code inside this repo and upgrade the version in the PR but do not modify src/index.json.

[yonzhan]

storage-blob-preview