This test had been intermittently failing due to a race condition. Its
implementation of `plugin.Provider.Read` was intended to ensure that
the cancellation of a refresh operation occurred. As written, it was
only able to ensure that the cancellation was requested.
These changes ensure that cancellation has been acknowledged by the engine by
implementing providing an implementation for `plugin.Provider.Cancel`
that closes a channel on which the implementation of `Read` waits.
3e65bc6517