Fix long expr-test time.

This disables optimizations while running expr-test based on a env var. Most of
our jenkins job will run with it disabled.

Change-Id: I680734a354e3ef4899cc626efed643ba2c9b5051
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3545
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>