Unit test support

[SchrodingersGat]

• Closes Unit Testing #12
• Ref: https://github.com/wolflu05/inventree-bulk-plugin

Tasks

• Add stubs for unit testing (no InvenTree integration)
• Add stubs for full testing (integration with InvenTree)
• Add stubs for playwright testing (with InvenTree integration)
• Run unit tests for github CI
• Run unit tests for gitlab CI
• Add documentation for writing unit tests for plugins
• Add "unit tests for unit tests" as part of CI for this plugin creator repo

[SchrodingersGat]

@wolflu05 I would love some advice / input on this, you've done a great job setting up unit tests for your plugin(s). I'd like to get that concept supported in the plugin creator tool too