Fix test suite bug for notwritable node.

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>
Fix AgentX testset response bug.

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>
Fix typo.

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>
Fix snmpset not writable response bug.

If a node is not exist when write, a "notWritable" shoule be responsed but not "noSuchObj".

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>