flatten-array: add ignore import statement [Fix #379]

exercises/flatten-array/src/test/java/FlattenerTest.java

@@ -1,5 +1,6 @@
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.Ignore;
 
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;