fix tests

Author: Sital Kedia

core/src/test/java/org/apache/spark/io/GenericFileInputStreamSuite.java

@@ -31,7 +31,7 @@
 /**
  * Tests functionality of {@link NioBufferedFileInputStream}
  */
-public class GenericFileInputStreamSuite {
+public abstract class GenericFileInputStreamSuite {
 
   private byte[] randomBytes;