PDFBOX-5017: use concurrency

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1886980 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/pdfbox/src/test/java/org/apache/pdfbox/pdmodel/graphics/image/PNGConverterTest.java b/pdfbox/src/test/java/org/apache/pdfbox/pdmodel/graphics/image/PNGConverterTest.java
index 33f7d10..eca8580 100644
--- a/pdfbox/src/test/java/org/apache/pdfbox/pdmodel/graphics/image/PNGConverterTest.java
+++ b/pdfbox/src/test/java/org/apache/pdfbox/pdmodel/graphics/image/PNGConverterTest.java
@@ -52,7 +52,10 @@
 
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.Execution;
+import org.junit.jupiter.api.parallel.ExecutionMode;
 
+@Execution(ExecutionMode.CONCURRENT)
 class PNGConverterTest
 {
     private static final File parentDir = new File("target/test-output/graphics/graphics");