SPARK-2798 [BUILD] Jenkins build failing due to missing scalatest in flume-sink module

The build is still not quite passing even after https://issues.apache.org/jira/browse/SPARK-2749 

See for example:

https://amplab.cs.berkeley.edu/jenkins/view/Spark/job/Spark-Master-Maven-pre-YARN/lastFailedBuild/hadoop.version=1.0.4,label=centos/console

It's the Flume Sink module that's failing, and I couldn't see the error until I ran it for myself and it seems to be:

```
Error: Could not find or load main class org.scalatest.tools.Runner
```
scalatest is a test dependency of other modules but not this one.

This PR has a change to just add the scalatest dep, but, also one to touch up a few other potential issues with this POM, like an Avro version mismatch.