IMPALA-2621: Fix flaky UNIX_TIMESTAMP() test
IMPALA-2621: Fix flaky UNIX_TIMESTAMP() test

Previously the test would expect the execution of an expr to be
less than 5 seconds. The test failed presumably because execution
on EC2 took longer. Now the expr result is expected to be between
the time just before the execution started and just after execution
finished.

Change-Id: I88271ab4bd0c3ec85e9cada98a3db00d501b0d2d
Reviewed-on: http://gerrit.cloudera.org:8080/1397
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Internal Jenkins

ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC