The `tests/` directory in core (and in some extensions packaged with core) are missing. This was introduced in [[ https://gerrit.wikimedia.org/r/#/c/123220/ | 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.