Constrain test_explain to run only on text/none table format

The tests expect to be run against text/none tables which causes failures
on exhaustive test runs.
I don't think it adds any extra coverage to run these tests against lzo 
format so added a constraint.

Change-Id: Ib0878e2ba84107c9df4499def304fe45ba4fe4b4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1884
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Lenni Kuff <lskuff@cloudera.com>