Add test case to catch the issue https://codereview.chromium.org/2642623002/ is trying to fix

<quote>
Suppose VAVDA got client request Decode()*N and then a Flush(), they
will be posted to decoder thread and handled by DecodeTask and
FlushTask.  Ideally DecodeTask will decode all N input, so no pending
input remains when FlushTask starts.

However, the DecodeTask() encountered a resolution change when it
processes one of input buffer. It has to allocate new surfaces. So it
post something to main thread and expect next DecodeTask will come back
to finish inputs. During the main thread allocating new surfaces, the
decode thread do next task in the queue --- FlushTask, which starts
before all input buffers are done.
</quote>