[SPARK-18671][SS][TEST-MAVEN] Follow up PR to fix test for Maven

## What changes were proposed in this pull request?

Maven compilation seem to not allow resource is sql/test to be easily referred to in kafka-0-10-sql tests. So moved the kafka-source-offset-version-2.1.0 from sql test resources to kafka-0-10-sql test resources.

## How was this patch tested?

Manually ran maven test

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #16183 from tdas/SPARK-18671-1.

external/kafka-0-10-sql/src/test/scala/org/apache/spark/sql/kafka010/KafkaSourceOffsetSuite.scala

@@ -98,7 +98,8 @@ class KafkaSourceOffsetSuite extends OffsetSuite with SharedSQLContext {
 
   private def readFromResource(file: String): SerializedOffset = {
     import scala.io.Source
-    val str = Source.fromFile(getClass.getResource(s"/structured-streaming/$file").toURI).mkString
+    val input = getClass.getResource(s"/$file").toURI
+    val str = Source.fromFile(input).mkString
     SerializedOffset(str)
   }
 }