The tests/ directory in core (and in some extensions packaged with core) are missing. This was introduced in Change-Id I341892c3ccfb3c2ac8eee565205c4c8164af5a72.

The tests are gone, but the code that looks for them is not. This means all unit-tests are broken in releases of MediaWiki. Not including the tests/ directory is wrong. Not only is the release incomplete, it is inconsistent -- there is code that breaks when run (references to files that do not exist.) In my opinion, a robust open source software package such as MediaWiki should be proud to ship its unit-tests. The 4MB of tests don't significantly contribute to the size of the release (some 80MB+.)

While some people might checkout the code from git to run the unit-tests, many do not. This doesn't fix the fact that the current code in MW core is broken in the releases.