I had to change some of the tests that used 4 tests rows, to have more (9),
to make them pass on a 3 segment cluster. If you only insert 4 rows to an
external table, some segments might not receive any rows, and reading from
the same file later will fail because the file wasn't created. I also
changed the queries that tested that rows are distributed evenly, to work
with a different number of segments.

Hook up the new tests to installcheck-world.