MTR tests fail when built without WSREP

When building with -DWITH_WSREP=OFF, files required for MTR tests are excluded and several tests fail. This is cause by a recent commit 7b44d0ba which attempted to solve MDEV-23230.

Failures can be seen in GitLab CI, or by building/installing/testing in the fedora:latest Docker container with the -DWITH_WSREP=OFF CMake flag enabled.

It appears this issue happened before (see !2196 and MDEV-28782) where WITH_WSREP=0 cause MTR tests to break.

Perhaps there should be a CI build with WSREP/Galera disabled to catch this mistake sooner.