The mcts testsuite is not distributed along with GCC due to unclear
distribution terms of its contents, which have to be clarified with
the CWI.

In the meanwhile, you can find the testset at:

  https://git.sr.ht/~jemarch/a68-mcts

In order to install them in the GCC sources, just copy the file and
two directories to the testsuite/algol68 directory, like:

  $ git clone https://git.sr.ht/~jemarch/a68-mcts
  $ cd a68-mcts
  $ cp README.mcts  /path/to/gcc/gcc/testsuite/algol68/
  $ cp -r compile/* /path/to/gcc/gcc/testsuite/algol68/compile/
  $ cp -r execute/* /path/to/gcc/gcc/testsuite/algol68/execute/

Then make check-algol68 will include the evil MCTS as well.