try to robustify test for bug-309448

try to robustify test for bug-309448

  We've been getting intermittent reports of it failing, but I've been unable
  to reproduce so far. The test in question is a compiler scaling test, which
  compares times it takes to compile different things. Brittle, indeed.

  Other than non-SBCL causes the only thing I can think of right now is GCs
  from other tests getting accounted against one of the supposed-to-be-fast
  runs. So, run GC :FULL T before getting the timings to make things more
  consistent.