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

@@ -135,6 +135,9 @@ public class TestSftpFileOutputPlugin
     @Rule
     public TemporaryFolder testFolder = new TemporaryFolder();
 
+    public String oldVfsSftpSshdir = null;
+    public String dotSshFolder = null;
+
     private static final ConfigMapperFactory CONFIG_MAPPER_FACTORY = SftpFileOutputPlugin.CONFIG_MAPPER_FACTORY;
     private static final ConfigMapper CONFIG_MAPPER = SftpFileOutputPlugin.CONFIG_MAPPER;
     private static final TaskMapper TASK_MAPPER = SftpFileOutputPlugin.TASK_MAPPER;
@@ -165,6 +168,10 @@ public class TestSftpFileOutputPlugin
     public void createResources()
             throws IOException
     {
+        this.dotSshFolder = testFolder.newFolder().getAbsolutePath();
+        this.oldVfsSftpSshdir = System.getProperty("vfs.sftp.sshdir");
+        System.setProperty("vfs.sftp.sshdir", this.dotSshFolder);
+
         // setup the plugin
         SftpFileOutputPlugin sftpFileOutputPlugin = new SftpFileOutputPlugin();
         runner = new FileOutputRunner(sftpFileOutputPlugin);
@@ -226,6 +233,14 @@ public void cleanup() throws InterruptedException
         catch (Exception e) {
             logger.debug(e.getMessage(), e);
         }
+        finally {
+            if (this.oldVfsSftpSshdir == null) {
+                System.clearProperty("vfs.sftp.sshdir");
+            }
+            else {
+                System.setProperty("vfs.sftp.sshdir", this.oldVfsSftpSshdir);
+            }
+        }
     }
 
     private List<String> lsR(List<String> fileNames, Path dir)