[SPARK-18283][STRUCTURED STREAMING][KAFKA] Added test to check whether default...

[SPARK-18283][STRUCTURED STREAMING][KAFKA] Added test to check whether default starting offset in latest

## What changes were proposed in this pull request?

Added test to check whether default starting offset in latest

## How was this patch tested?
new unit test

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #15778 from tdas/SPARK-18283.

(cherry picked from commit b06c23db)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>