Point me to some open source projects *with* (unit) tests

OpenJDK has a very comprehensive set of conformance tests that are required to pass in order to be considered a valid implementation of Java. :-)

SQLite has a very comprehensive test suite. It may not be unit testing, per se, but it is quite comprehensive.

As of version 3.6.16 (all statistics in the report are against that release of SQLite), the SQLite library consists of approximately 63.9 KSLOC of C code. (KSLOC means thousands of "Source Lines Of Code" or, in other words, lines of code excluding blank lines and comments.) By comparison, the project has 709 times as much test code and test scripts - 45385.9 KSLOC.