Broken regression test PartitionedFilterBlockTest.PrefixInWrongPartitionBug

[pdillinger]

In some recent dev work I discovered that breaking the behavior this test is supposed to check for (details in 6652c94) is no longer caught by the test. And I drafted a change (https://github.com/pdillinger/rocksdb/commit/0a7af0d6a55c219d40898f87f6db3361bacfb531) showing that using the old version of the test immediately before 9f7877f, naively ported to latest code, does catch the broken behavior. Apparently, the test changes in 9f7877f mean it is no longer checking what it was originally checking.