add a test case for DataFile write and read

skywalking-storage-center/skywalking-storage/src/test/java/com/a/eye/skywalking/storage/data/file/DataFileTest.java

+package com.a.eye.skywalking.storage.data.file;
+
+import com.a.eye.skywalking.network.grpc.RequestSpan;
+import com.a.eye.skywalking.storage.data.index.IndexMetaInfo;
+import com.a.eye.skywalking.storage.data.spandata.RequestSpanData;
+import com.a.eye.skywalking.storage.data.spandata.SpanDataBuilder;
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Created by wusheng on 2016/11/29.
+ */
+public class DataFileTest {
+    @Test
+    public void testWriteFile(){
+        DataFile dataFile = new DataFile();
+
+        IndexMetaInfo info = null;
+        for (int i = 0; i < 100; i++) {
+            RequestSpan span = RequestSpan.newBuilder().setUserId("1").setApplicationId("app").build();
+
+            try {
+                info = dataFile.write(new RequestSpanData(span));
+            } finally {
+                dataFile.flush();
+            }
+
+            RequestSpan newSpan = SpanDataBuilder.buildRequestSpan(dataFile.read(info.getOffset(), info.getLength()));
+
+            Assert.assertEquals("1", newSpan.getUserId());
+        }
+    }
+}