tests: Create a src/tests/ symlink that points to tests/

This makes it possible for plugin backend tests to do
`require('ep_etherpad-lite/tests/backend/common')` to access the API
key (among other things).

Eventually we probably should reverse these (move `tests/` to
`src/tests/` and make `tests/` a symlink to `src/tests/`) and move
`bin/` to `src/bin/` so that we can avoid the top-level `package.json`
mess.

src/tests

@@ -0,0 +1 @@
+../tests