Add extensive tests for partition pruning.

Currently, partition pruning happens via constraint exclusion, but
there are pending places to replace that with a different and
hopefully faster mechanism.  To be sure that we don't change behavior
without realizing it, add extensive test coverage.

Note that not all of these behaviors are optimal; in some cases,
partitions are not pruned even though it would be safe to do so.
These tests therefore serve to memorialize the current state rather
than the ideal state.  Patches that improve things can update the test
results as appropriate.

Amit Langote, adjusted by me.  Review and testing of the larger patch
set of which this is a part by Ashutosh Bapat, David Rowley, Dilip
Kumar, Jesper Pedersen, Rajkumar Raghuwanshi, Beena Emerson, Amul Sul,
and Kyotaro Horiguchi.

Discussion: http://postgr.es/m/098b9c71-1915-1a2a-8d52-1a7a50ce79e8@lab.ntt.co.jp