* src/ltlvisit/randomltl.cc: Fix generation of formulas when unary or
binary operators are missing.
* src/ltlvisit/apcollect.cc, src/ltlvisit/apcollect.hh
(destroy_atomic_prop_set): New function.
* src/bin/randltl.cc: Use it, and also honnor --boolean-priorities
when generating SEREs.
* src/ltltest/rand.test: New file.
* src/ltltest/Makefile.am: Add it.