Replace static fixture by generated one in issue_spec.js

## What does this MR do?

- clean up `issue_spec.js`
- introduce an alternative approach to #19445
  - rename `rake teaspoon` to `rake teaspoon:tests`
  - introduce `rake teaspoon:fixtures` which generates fixtures using RSpec
  - introduce `rake teaspoon` which runs `rake teaspoon:fixtures` and `rake teaspoon:tests`

## Why was this MR needed?

- many duplications
- missing existence checks
- missing conditions
- static fixtures don't match real views

## Reasoning

I want to explain some of my decisions here, so that they stay visible for future discussions.

### Why not HAML?

- same number of HAML templates as number of fixtures (many input files)
- embedded logic less readable
- can not be rendered by JavaScript (because of inline Ruby)

### Why RSpec?

- real controllers for fixtures
- spys available for mocking
- easily report failed fixture generations

### Why not magic_lamp? (#19445)

- introduces another dependency/tool
- needs to run a server concurrently to teaspoon
- makes it harder to use a JavaScript test runner
- static HTML files serve faster

See merge request !6059