developer-guide.md: add new section 'Unit-test writing' to all languages

New section was introduced by 9bc3a8c2. This PR add it to all languages.

also add the missing 'developer-guide.md' file in /sv

changed the description

Thank you for doing this @erciccione. Can you please rebase against current master (and re-copy the file over to the other languages)? I've pushed b52fa74c and 4eb5654b.

added 3 commits

• 05e609ce...4eb5654b - 2 commits from branch kovri-project:master
• 6d5c1964 - developer-guide.md: add new section 'Unit-test writing' to all languages

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@anonimal rebased and fixed

added 1 commit

• 2118b317 - change encoding of file in /nl to UTF-8

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added 1 commit

• 05e609ce - developer-guide.md: add new section 'Unit-test writing' to all languages

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Pushed some commits on this branch by mistake. Now fixed and still good to go

@erciccione Is this good to merge with the fix?

assigned to @seanfroth

Yes, it's good to go @seanfroth

added 7 commits

• 05e609ce...5ab16f3a - 6 commits from branch kovri-project:master
• df3a3598 - Merge branch 'master' into 'new-sec'

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merged