Fix environment-dependent test failures for the default ssh directory…

… of Apache Commons VFS

https://github.com/apache/commons-vfs/blob/rel/commons-vfs-2.9.0/commons-vfs2/src/main/java/org/apache/commons/vfs2/provider/sftp/SftpClientFactory.java#L281-L293

src/test/java/org/embulk/output/sftp/TestSftpFileOutputPlugin.java

@@ -165,6 +165,7 @@ public class TestSftpFileOutputPlugin
     public void createResources()
             throws IOException
     {
+        System.setProperty("vfs.sftp.sshdir", testFolder.newFolder("dotssh").getAbsolutePath());
         // setup the plugin
         SftpFileOutputPlugin sftpFileOutputPlugin = new SftpFileOutputPlugin();
         runner = new FileOutputRunner(sftpFileOutputPlugin);