feat(test): add completion tests with applying in editor

[xpyctumo]

Resolves: #275

As an example, I took all the existing tests for completions and duplicated them, but, now, the tests choose the most relevant option for completion (depending on the beginning of the string).

How completions are applied:

• If one exists, the template is applied.
• If there are no suitable variants, the first available variant in the code is applied.

Want to do:

• It might be worth working on auto-duplicating tests in the future