Clean up buffer clean up/fill logic

The alt buffer is now cleared immediated after activating the normal buffer and
is filled when switching to it. The tests were failing because the alt buffer
wasn't being cleared properly with the previous solution.

src/utils/CircularList.ts

@@ -5,8 +5,9 @@
  * @license MIT
  */
 import { EventEmitter } from '../EventEmitter';
+import { ICircularList } from '../Interfaces';
 
-export class CircularList<T> extends EventEmitter {
+export class CircularList<T> extends EventEmitter implements ICircularList<T> {
   private _array: T[];
   private _startIndex: number;
   private _length: number;

src/utils/TestUtils.ts

@@ -0,0 +1,53 @@
+import { ITerminal, IBuffer, IBufferSet, IBrowser, ICharMeasure, ISelectionManager } from '../Interfaces';
+
+export class MockTerminal implements ITerminal {
+  public element: HTMLElement;
+  public rowContainer: HTMLElement;
+  public selectionContainer: HTMLElement;
+  public selectionManager: ISelectionManager;
+  public charMeasure: ICharMeasure;
+  public textarea: HTMLTextAreaElement;
+  public rows: number;
+  public cols: number;
+  public browser: IBrowser;
+  public writeBuffer: string[];
+  public children: HTMLElement[];
+  public cursorHidden: boolean;
+  public cursorState: number;
+  public defAttr: number;
+  public scrollback: number;
+  public buffers: IBufferSet;
+  public buffer: IBuffer;
+
+  handler(data: string) {
+    throw new Error('Method not implemented.');
+  }
+  on(event: string, callback: () => void) {
+    throw new Error('Method not implemented.');
+  }
+  scrollDisp(disp: number, suppressScrollEvent: boolean) {
+    throw new Error('Method not implemented.');
+  }
+  cancel(ev: Event, force?: boolean) {
+    throw new Error('Method not implemented.');
+  }
+  log(text: string): void {
+    throw new Error('Method not implemented.');
+  }
+  emit(event: string, data: any) {
+    throw new Error('Method not implemented.');
+  }
+  reset(): void {
+    throw new Error('Method not implemented.');
+  }
+  showCursor(): void {
+    throw new Error('Method not implemented.');
+  }
+  blankLine(cur?: boolean, isWrapped?: boolean) {
+    const line = [];
+    for (let i = 0; i < this.cols; i++) {
+      line.push([0, ' ', 1]);
+    }
+    return line;
+  }
+}