Set prefix to 'junit-' in standard TempDirFactory (#3806)

To improve readability of generated directory names.

junit-jupiter-api/src/main/java/org/junit/jupiter/api/io/TempDirFactory.java

@@ -81,7 +81,7 @@ class Standard implements TempDirFactory {
 
 		public static final TempDirFactory INSTANCE = new Standard();
 
-		private static final String TEMP_DIR_PREFIX = "junit";
+		private static final String TEMP_DIR_PREFIX = "junit-";
 
 		public Standard() {
 		}

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/extension/TempDirectoryPerContextTests.java

@@ -697,7 +697,7 @@ private static class Factory implements TempDirFactory {
 			@Override
 			public Path createTempDirectory(AnnotatedElementContext elementContext, ExtensionContext extensionContext)
 					throws Exception {
-				return Files.createTempDirectory("junit");
+				return Files.createTempDirectory("junit-");
 			}
 		}