[SPARK-18730] Post Jenkins test report page instead of the full console output page to GitHub

## What changes were proposed in this pull request?

Currently, the full console output page of a Spark Jenkins PR build can be as large as several megabytes. It takes a relatively long time to load and may even freeze the browser for quite a while.

This PR makes the build script to post the test report page link to GitHub instead. The test report page is way more concise and is usually the first page I'd like to check when investigating a Jenkins build failure.

Note that for builds that a test report is not available (ongoing builds and builds that fail before test execution), the test report link automatically redirects to the build page.

## How was this patch tested?

N/A.

Author: Cheng Lian <lian@databricks.com>

Closes #16163 from liancheng/jenkins-test-report.

(cherry picked from commit ba4aab9b)
Signed-off-by: Reynold Xin <rxin@databricks.com>