@Override
              public void run() {
                threadStartLatch.countDown();
                try {
                  channel.write(ByteBuffer.allocate(20));
                  interruptException.set(null);
                } catch (Throwable e) {
                  interruptException.set(e);
                }
              }
Most of the behavior of {@link JimfsFileChannel} is handled by the {@link RegularFile}
implementations, so the thorough tests of that are in {@link RegularFileTest}. This mostly tests
interactions with the file and channel positions.

@author Colin Decker
jimfs/src/test/java/com/google/common/jimfs/JimfsFileChannelTest.java