Add tests setup

[fredrikaverpil]

This adds a test set up for pure lua functions as well as runs neotest-golang against go code.

To do:

• [x] Rename test to tests.
• [ ] Add go formatter (golines, gofumpt...) - could also be done in separate PR.
• [x] Clean up installer shell script.
• [x] Clean up minimal_init.lua.
• [x] Add gomod to dependabot.