test_icmpv6: Catch proper Exception
test_icmpv6: Catch proper Exception

In some test cases in test_icmpv6.py, @raises() decorator does not catch
the correct Exception, because "pack" function cannot be resolved and it
need to be specified as "struct.pack".

This patch fixes to this unresolved references and make it more strictly
matching of Exception class (e.g., AssertionError).

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

iwaseyusuke/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,iwaseyusuke/ryu,iwaseyusuke/ryu,osrg/ryu,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,osrg/ryu,osrg/ryu,lagopus/ryu-lagopus-ext,lagopus/ryu-lagopus-ext,fujita/ryu,osrg/ryu,lagopus/ryu-lagopus-ext,fujita/ryu,lagopus/ryu-lagopus-ext,fujita/ryu,fujita/ryu