Create a test case for the poll loop which uses the loop to read data from a

Create a test case for the poll loop which uses the loop to read data from a
pipe and assert that the data written to the pipe is identical to the data
read from the pipe. In order to implement this test, several useful classes
have been added:

 * PipeReader

     Reads output from one or more files and saves it in memory,
     for retrieval via the getvalue() method. This is driven by
     the scheduler's poll() loop, so it runs entirely within the
     current process.

 * QueueScheduler

     Add instances of SequentialTaskQueue and then call run().
     The run() method returns when no tasks remain.

 * TaskScheduler

     A simple way to handle scheduling of AsynchrousTask instances. Simply
     add tasks and call run(). The run() method returns when no tasks remain.

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