[SPARK-20455][DOCS] Fix Broken Docker IT Docs

## What changes were proposed in this pull request?

Just added the Maven `test`goal.

## How was this patch tested?

No test needed, just a trivial documentation fix.

Author: Armin Braun <me@obrown.io>

Closes #17756 from original-brownbear/SPARK-20455.

(cherry picked from commit c8f12195)
Signed-off-by: Sean Owen <sowen@cloudera.com>