Bug 1254 - OFP test coverage increase

- ByteUtil is final with private constructor instead of being abstract
- ByteUtil#bytesToUnsignedInt expects its paramater to be exactly 4 bytes long
- ByteUtil#bytesToUnsignedShort expects its parameter to be exactly 2 bytes long
- ByteUtil tests added

Change-Id: I7820bb0e0c33070317c08c1f3fd9a65ce99f6f09
Signed-off-by: Martin Bobak <mbobak@cisco.com>