@Test
  public void testTransferFrom() throws IOException {
    RegularFile file = regularFile(0);
    FileChannel channel = channel(file, WRITE);

    ByteBufferChannel readChannel = new ByteBufferChannel(buffer("1234567890"));
    assertEquals(10, channel.transferFrom(readChannel, 0, 100));
    assertEquals(0, channel.position());
  }
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