[SPARK-17710][HOTFIX] Fix ClassCircularityError in ReplSuite tests in Maven build: use 'Class.forName' instead of 'Utils.classForName'

## What changes were proposed in this pull request?
Fix ClassCircularityError in ReplSuite tests when Spark is built by Maven build.

## How was this patch tested?
(1)
```
build/mvn -DskipTests -Phadoop-2.3 -Pyarn -Phive -Phive-thriftserver -Pkinesis-asl -Pmesos clean package
```
Then test:
```
build/mvn -Dtest=none -DwildcardSuites=org.apache.spark.repl.ReplSuite test
```
ReplSuite tests passed

(2)
Manual Tests against some Spark applications in Yarn client mode and Yarn cluster mode. Need to check if spark caller contexts are written into HDFS hdfs-audit.log and Yarn RM audit log successfully.

Author: Weiqing Yang <yangweiqing001@gmail.com>

Closes #15286 from Sherry302/SPARK-16757.