~SleepingBackgroundTask() to wake up the sleeping task (#11036)

~SleepingBackgroundTask() to wake up the sleeping task (#11036)

Summary:
Right now, in unit tests, when background tests are sleeping using SleepingBackgroundTask, and the test exits with test assertion failure, the process will hang and it might prevent us to see the test failure message in CI runs. Try to wake up the thread so that the test can exit correctly.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11036

Test Plan: Watch CI succeeds

Reviewed By: riversand963

Differential Revision: D42020489

fbshipit-source-id: 5b8441b18d5f67bbb3ade59a1225a8d3c860c2eb