Fix the ASAN build by xfailing test_decimal when ASAN_OPTIONS is set.

Adding decimal columns crashes an ASAN built impalad. This change skips the test.

Change-Id: Ic94055a3f0d00f89354177de18bc27d2f4cecec2
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2532
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins