Fix generation of random formulae on 64bits systems.

* src/main.cc (testLoop): Generate random short ints between 0 and
SHRT_MAX, not between 0 and LONG_MAX.  On systems where long ints
are 64bits, LRAND(0,LONG_MAX) was returning a value with the lower
32 bits set to 0, and the latter truncation to short int always
yielded the value 0.  Consequently all generated formulae were
identical...