HADOOP-13230 factoring out of operation cost assertions

- move constants of head/list costs into its own class for use
  across tests

- move the declarative assertions about op costs into its own class,
  OperationCostValidator

This is part of a change to move the new operation cost tests into
their own suite, as this one is too big and becoming too much
merge conflict. Also it'd be nice to include cost asserts into
other test suites.

Change-Id: Ifafa22e5b66a1fc27f61e81993c3b642a0210d1c

Author: steveloughran