isolationtester seems uselessly rigid as to length of permutation

I thought it'd be a good idea to put in some basic test cases for the
EvalPlanQual code using the isolationtester infrastructure. While
fooling with it, I soon ran into this restriction:

if (p->nsteps != nallsteps)
{
fprintf(stderr, "invalid number of steps in permutation %d\n", i + 1);
exit_nicely();
}

ie, a "permutation" list has to specify exactly as many steps as there
are in the spec file. This seems to me to be a useless restriction,
because it prevents running a test sequence that repeats some steps or
leaves other steps out. Barring objections, I'm going to remove the
above lines.

regards, tom lane