@Test
  public void testPosition() throws IOException {
    FileChannel channel = channel(regularFile(10), READ);
    assertEquals(0, channel.position());
    assertSame(channel, channel.position(100));
    assertEquals(100, 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