Adjust _NHWCToNCHW and _NCHWToNHWC in bias_op_test.py

The _NHWCToNCHW and _NCHWToNHWC in bias_op_test.py
only took into considerations about 4D situation,
as it assume that third-to-last dim is the channel
dimension.

This fix adjust to use second-to-first dim as the channel
dimension so that 3D/4D/5D data could be handled by NCHW format.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>