The unittests_*.yml test suites were added in SERVER-18839 to make it easier to run the C++ unit tests related to a particular component locally. However, they can no longer be meaningfully run after the unit tests executables were collapsed into a single executable per-directory as part of the changes from SERVER-41809. They are doubly unable to be meaningfully run as a result of the switch to hygienic builds due to how build/unittests.txt refers to the files as being in the build/install/bin/ directory.