[ZEPPELIN-6254] Add test case in file interpreter

### What is this PR for?
Added test cases to the file interpreter to increase test coverage.

### What type of PR is it?
Improvement

### Todos
* command is null
* use unknown command
* no such directory/file

### What is the Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-6254

### How should this be tested?
Run test in file interpreter

### Screenshots (if appropriate)

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No


Closes #4994 from proceane/feature/ZEPPELIN-6254.

Signed-off-by: Philipp Dallig <philipp.dallig@gmail.com>

Author: proceane

file/src/test/java/org/apache/zeppelin/file/HDFSFileInterpreterTest.java

@@ -133,6 +133,56 @@ void test() {
 
     t.close();
   }
+
+  @Test
+  void testCommandIsNull() {
+    HDFSFileInterpreter t = new MockHDFSFileInterpreter(new Properties());
+    t.open();
+
+    InterpreterResult result = t.interpret(null, null);
+    assertEquals(InterpreterResult.Type.TEXT, result.message().get(0).getType());
+    assertEquals("No command", result.message().get(0).getData());
+
+    t.close();
+  }
+
+  @Test
+  void testUnknownCommand() {
+    HDFSFileInterpreter t = new MockHDFSFileInterpreter(new Properties());
+    t.open();
+
+    InterpreterResult result = t.interpret("unknown", null);
+    assertEquals(InterpreterResult.Type.TEXT, result.message().get(0).getType());
+    assertEquals("Unknown command", result.message().get(0).getData());
+
+    t.close();
+  }
+
+  @Test
+  void testNoSuchDirectory() {
+    HDFSFileInterpreter t = new MockHDFSFileInterpreter(new Properties());
+    t.open();
+
+    InterpreterResult result = t.interpret("cd /tmp/ida8c06540_date040315", null);
+    assertEquals(InterpreterResult.Type.TEXT, result.message().get(0).getType());
+    assertEquals("/tmp/ida8c06540_date040315: No such directory",
+            result.message().get(0).getData());
+
+    t.close();
+  }
+
+  @Test
+  void testNoSuchFile() {
+    HDFSFileInterpreter t = new MockHDFSFileInterpreter(new Properties());
+    t.open();
+
+    InterpreterResult result = t.interpret("ls -l /does/not/exist", null);
+    assertEquals(InterpreterResult.Type.TEXT, result.message().get(0).getType());
+    assertEquals("No such File or directory", result.message().get(0).getData());
+
+    t.close();
+  }
+
 }
 
 /**