When running test/langtools/tools/javac/warnings/Serial.java with `-retain:all`, the ".jtr" files inside the `Serial` directory (e.g., test/langtools/tools/javac/warnings/Serial/Extern.jtr) are deleted. This issue also occurs for test/langtools/tools/javac/generics/ParametricException.java (on case-insensitive file systems) and test/jdk/java/awt/dnd/URLDragTest.java.

This issue occurs when there is a test directory whose name is the same as the test name (without extension).
This behaviour is similar to that reported in https://bugs.openjdk.org/browse/JDK-8318689, and a similar fix should be applied.