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

i18n/ar/developer_guide.md

@@ -139,11 +139,11 @@ cases and failure csaes.
 the test should verify that the documented behavior is implemented, but
 should not break if other permissible behavior is later implemented.
 
-In addition to not requiring a network connection, *unit-tests* should not require socket or filesystem access unless the test is socket/filesystem-specific test (these are unit-tests, not inteegration tests).
+In addition to not requiring a network connection, *unit-tests* should not require socket or filesystem access unless the test is socket/filesystem-specific test (these are unit-tests, not integration tests).
 
 Other notes:
 
-- Though we have a Docker testnet, we currenly lack any effective framework for integration and system testing. As such, the best we can do at the moment is effective unit testing.
+- Though we have a Docker testnet and Boost.Python hooks, our framework for integration and system testing are a WIP. As such, the best we can do at the moment is effective unit testing.
 - For gcov output when building tests, build with `make coverage`. This target should also build unit-tests.
 - For existing kovri examples, see `crypto/{ed25519.cc,radix.cc}` and `util/buffer.cc` to name a few
 - For effective unit-test writing outside of Tor, see Crypto++ and Monero unit-tests