Add completion tests for class/module body contexts

[Morriar]

Context

Completion is incorrect in class/module bodies. After Shopify/rubydex#794 lands and the Ruby LSP passes self_receiver, we need comprehensive test coverage for this area.

Related: #4106

Tasks

• Add completion tests for constants inside class/module bodies
• Add completion tests for class variables inside class/module bodies
• Add completion tests for methods inside class/module bodies
• Add completion tests for instance variables inside class/module bodies
• Verify combinations of the above in nested contexts