Test case to check getsockopt operation SO_COOKIE

Added a test case to get the socket cookie using getsockopt and verify
the result by comparing it with the cookie retrieved by sock_diag msg.
It is used to make sure the kernel have the corresponding patches for
SO_COOKIE option.

Test: new unit test passes on kernel 4.9 and above

Signed-off-by: Chenbo Feng <fengc@google.com>
Bug: 30950746
Change-Id: Iaa7a450bf390a454969df5d46b0b23d2ed640be8