Added tests for Greenplum

Added a script `test/test_gp.sh` that run deepdive tests under a GP settings. When running `test.sh`, If `gpfdist` command is present, the script will be executed.

Now we run following tests for GP:
- unit tests (without parallel grounding)
- biased coin (with / without parallel grounding)
- spouse example (with / without parallel grounding)

For chunking, there are some file permission issues that I haven't been able to resolve yet. (so both tests with / without parallel grounding fails)

If it's OK to skip chunking example in the GP test, we can set up the automatic test pretty soon.

Good point. We went over it in the past. Ok, I'm going to merge.