SPARK-2798 [BUILD] Correct several small errors in Flume module pom.xml files

(EDIT) Since the scalatest issue was since resolved, this is now about a few small problems in the Flume Sink `pom.xml`

- `scalatest` is not declared as a test-scope dependency
- Its Avro version doesn't match the rest of the build
- Its Flume version is not synced with the other Flume module
- The other Flume module declares its dependency on Flume Sink slightly incorrectly, hard-coding the Scala 2.10 version
- It depends on Scala Lang directly, which it shouldn't
Through spark-streaming then? take a look at `mvn dependency:tree -pl external/flume-sink/`; it is included. It is not a big deal now that the dependency here is not on a fixed version, but inherits from the parent. But it seems that most modules do not declare this themselves.