Multiple setup steps for isolation tests

I just today found that the index-only scan feature has broken SSI.
I don't think it will take much to fix, and I'm looking at that, but
the first thing I wanted was a test to show the breakage. I couldn't
find a way to do that without running VACUUM after loading data to
the test tables, and because VACUUM refuses to run in a
multi-statement batch I propose the following patch to the isolation
testing code, which allows multiple setup blocks. Using this code I
now have an isolation test to show the breakage.

If there are no objections, I will apply this to HEAD and 9.2.

I'm working on a fix to the bug itself.

-Kevin