The tests/ directory in core (and in some extensions packaged with core) are missing. This was introduced in
change 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. Including the unit-testing registration but not including the tests/ directory is wrong. In this state the code is not only is the incomplete, it is inconsistent -- there is code that breaks when run (references to files that do not exist.) Without these files no-one can run any unit-tests.

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.