Automated test and improve test coverage

[noklam]

Context:
There are only very few tests implemented at the moment since the features are changed and tested manually all the time. As it starts to stabilised we should aim at setting up test regularly.

Current test folder:

• https://github.com/kedro-org/vscode-kedro/tree/main/src/test/python_tests

Instead of aiming at 100% coverage, it will be more beneficial to test certain behavior in a blackbox manner.

• The extension is not a library, so risk of breaking a production codebase is low
• Most people would upgrade it to the latest version (similar to most people upgrade kedro-viz to latest version because it's an application)