"Go to test" button in merge request review
### Problem to solve
When reviewing an MR I'm very often jumping between a class and then the test for that class but it takes me some time to find the test in the side bar and vice versa.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
### User experience goal
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### Proposal
Some IDEs have a shortcut to simply jump to the test which matches the class/file you are looking at. Usually this is done by file name convention. For example in Ruby you just take the file name and look for the same file name with `_spec.rb` instead of `.rb` on the end inside the `spec/` directory. This is a heuristic but it works well enough to be useful.
In VIM for example there [are plugins](https://github.com/tpope/vim-projectionist#alternate-files) that know about file name conventions to jump between "alternate" files which are pairs of class and test files since this is a very common workflow especially in TDD.
### Further details
This was @cablett's awesome idea noted when doing some code review about how she misses this feature of Ruby Mine while doing code review.
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### Availability & Testing
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* Unit test changes
* Integration test changes
* End-to-end test change
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### What does success look like, and how can we measure that?
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### Is this a cross-stage feature?
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### Links / references
issue