Clock::advance can race with process::delay in tests.

There are lots of tests that use the following pattern:
1) Pause clocks
2) Start an agent
3) Advance clocks to trigger an event
4) Wait for the event

If an event is scheduled via `process::delay()` after advancing the clocks, then a test hangs in the endless wait for the event that is never triggered, because libprocess clocks are paused. For example, `DiskResource/PersistentVolumeTest.SharedPersistentVolumeRescindOnDestroy/0` test hangs at step 4, because the clocks at step 3 has been already advanced before the agent scheduled a call of Slave::authenticate() method. After a successful authentication with a master, the agent sends a UpdateSlaveMessage. But the authentication process never finishes because `Slave::authenticate()` is never called.

A list of tests that might be affected by the issue attached to this ticket.