MAPREDUCE-7310. Clear the fileMap in JHEventHandlerForSigtermTest (#2499)

Co-authored-by: Zhengxi Li <zli89@illinois.edu>
Reviewed-by: Wanqiang Ji <jiwq@apache.org>
Signed-off-by: Akira Ajisaka <aajisaka@apache.org>

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-app/src/test/java/org/apache/hadoop/mapreduce/jobhistory/TestJobHistoryEventHandler.java

@@ -1119,6 +1119,7 @@ class JHEvenHandlerForTest extends JobHistoryEventHandler {
 class JHEventHandlerForSigtermTest extends JobHistoryEventHandler {
   public JHEventHandlerForSigtermTest(AppContext context, int startCount) {
     super(context, startCount);
+    JobHistoryEventHandler.fileMap.clear();
   }
 
   public void addToFileMap(JobId jobId) {