More refactoring and improvement of skiplist tests.
Now Slm3 is still exhibiting issue #49, BUT I notice that it
is highly nondeterministic.  The same slm3 test will sometimes
take 0.10 seconds and sometimes 4.5 seconds.

The new sliceCheck shouldn't perturb the timing too much, because it's
after the join point.  I'm not sure why the behavior is different in this revision.