Download testgen.l, symtab.[ch], and auxiliary stuff:
wget -r --no-parent -nH http://mmix.cs.hm.edu/mmixmasters/tools/
Install system package flex: sudo apt install flex.
Step into mmixmasters/tools/ and run make testgen (or any of the other
targets in Makefile).
Download wget -r --no-parent -nH http://mmix.cs.hm.edu/supplement/.
Step into the individual example directories under supplement and invoke
../../mmixmasters/tools/testgen *.tst
mmixal -b 250 *.mms
mmix *.mmo
[You will have to increase -b 270 for 5.2.5/{10,11}-ex15.tst,mms because of extra-long lines.]