Add UI tests

[brichet]

This PR adds first UI tests, including:

• test the chat panel (button, opening, welcome message)
• error message if there is no AI provider
• AI response if the AI provider is set
• AI code completion if the AI provider is set

Fixes #67

[brichet]

The test on code completion seems to work well, but I wonder how stable it will be in the future.
Currently it checks if the ghost text is visible, and also if the request has been send.
If the ghost text fails too often, we can remove the ghost text check.

[jtpio]

The test on code completion seems to work well, but I wonder how stable it will be in the future.
Currently it checks if the ghost text is visible, and also if the request has been send.
If the ghost text fails too often, we can remove the ghost text check.

yeah that's also what I was wondering. Otherwise maybe we would need to trigger the completion several time and check that it shows up at least once?

[jtpio]

Thanks!