feat: change backgroundTasksFinished method from private to public

[CodyTseng]

Checklist

• [ ] npm test passes
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message follows commit guidelines

Affected core subsystem(s)

Description of change

有时候需要测试 background tasks 执行完后的结果，将此方法改为 public 会比较方便

[CodyTseng]

测试的错误是 tegg 的改动导致的 https://github.com/eggjs/tegg/pull/134/files#diff-97efa9b06dbc5312f138edf04f2e93055be52ce908ee060f2dabbbb6d575e941

简单看了下好像是 tegg 不允许调用两次 mockModuleContext，与 egg-mock 的 inject_context 功能相矛盾了？不知道我的猜测是否正确，仅供参考。