Increase completion's test coverage and add some minor improvements

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Changes

1. Add tests to cover primitive types' completion
2. Group some completion tests together
3. Evaluate completion candidates only when we really need them (usually when doc_namespace == false)
4. Correct assert_equal's argument order in completion tests (I'll go through other tests in follow up PRs)
5. Sort constant completion's candidates like Fixed sorting of variables and matched suggestions in completion #379 does

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cc @peterzhu2118

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@peterzhu2118 thank you ❤️