Enhancement

Currently TiFlash Compute engine's unit tests relay on TiFlashTestEnv::global_context to initialize the context that will be used in the test.
But in TiFlashTestEnv::global_context, we have initialized some components only related to storage layer.
And in gtests_dbms, we have also initialized some storage components.

    DB::ServerInfo server_info;
    // `DMFileReaderPool` should be constructed before and destructed after `SegmentReaderPoolManager`.
    DB::DM::DMFileReaderPool::instance();
    DB::DM::SegmentReaderPoolManager::instance().init(server_info);
    DB::DM::SegmentReadTaskScheduler::instance();

As a result, the UT will start extra threads using BackgroundProcessingPool , SegmentReaderPool, DMFileReaderPool etc.
We want to separate the init procedure of Compute Layer UT and Storage Layer UT in order to make the test process cleaner.