Temporarily disable explain tests for explain levels 2 and 3

The explain tests that verify we detect missing stats properly is failing for avro. This
change disables the test to unblock the full data load build.

Change-Id: I0a7f54dbf1e8a3ebb557250287e7e0491aaa27f2
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1924
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins