added test to verify new functionality

Author: teslae1

test/mode/modeVisual.test.ts

@@ -23,6 +23,13 @@ suite('Mode Visual', () => {
     assert.strictEqual(modeHandler.vimState.currentMode, Mode.Normal);
   });
 
+  newTest({
+    title: 'gv while in visual mode exhanges the previous visual area with current area',
+    start: ['|abcd'],
+    keysPressed: 'vyllvgvlp',
+    end: ['|acd'],
+  });
+
   newTest({
     title: '[count]v',
     start: ['a|bcde'],