{CI} Modify the extension's azdev test to use the azext_xxx directory

[wangzelin007]

Modify the extension's azdev test to use the azext_xxx directory to avoid the issue of being unable to detect the test when it has the same name as the main module.
The containerapp module exists in both the main repo and the extension repo.
After installing the extension, running azdev test containerapp will not run any tests. The log is as follows:

To avoid this problem, modify the azdev test $extension command to the azdev test azext_$extension command.

Test screenshot:

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

Related command

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? (pip install wheel==0.30.0 required)

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 pull request is merged into main branch, a new pull request will be created to update src/index.json automatically.
You only need to update the version information in file setup.py and historical information in file HISTORY.rst in your PR but do not modify src/index.json.

[azure-client-tools-bot-prd]

Hi @wangzelin007,
Please write the description of changes which can be perceived by customers into HISTORY.rst.
If you want to release a new extension version, please update the version in setup.py as well.

[azure-client-tools-bot-prd]

️✔️Azure CLI Extensions Breaking Change Test

️✔️Non Breaking Changes

[yonzhan]

CI