SerializeAddon: move test util into seperate file

addons/xterm-addon-serialize/test/SerializeAddonTestUtil.ts

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) 2021 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { IBuffer } from 'xterm';
+export class SerializeAddonTestUtil {
+  // this is a util used only for test
+  public static inspectBuffer(buffer: IBuffer): { x: number, y: number, data: any[][] } {
+    const lines: any[] = [];
+
+    for (let i = 0; i < buffer.length; i++) {
+      /**
+       * Do this intentionally to get content of underlining source
+       */
+      const bufferLine = (buffer.getLine(i)! as any)._line;
+
+      lines.push(JSON.stringify(bufferLine));
+    }
+
+    return {
+      x: buffer.cursorX,
+      y: buffer.cursorY,
+      data: lines
+    };
+  }
+}