Fix race condition vulnerability in temporary directory creation

[hsuk04]

Description

This PR modernizes thecreateTmpDir() method in ExporterTestUtils.java by replacing the older implementation with the Java NIO.2 Files API.
The previous implementation has a potential race condition vulnerability between deleting the temp file and creating a directory with the same name plus suffix. During this window, another process could potentially create a file or directory at the target path.
The new implementation using Files.createTempDirectory() eliminates this vulnerability by performing the operation atomically.

A similar fix can be found here openkm/document-management-system#332

Changes

• Replace the multi-step temp directory creation process with a single call to Files.createTempDirectory()
• Add import for java.nio.file.Files

References

openkm/document-management-system#332
openkm/document-management-system@c069e4d