zebra: fix unit test failure on non linux
zebra: fix unit test failure on non linux

syscall.AF_INET/AF_INET6 are os/architecture specific. On some non
Linux x86 architectures, failures like the following happen:

--- FAIL: Test_NexthopRegisterBody (0.00s)
        Error Trace:    zapi_test.go:893
        Error:			Not equal: 0x1e (expected)
        			        != 0xa (actual)

        Error Trace:    zapi_test.go:894
        Error:			Not equal: net.IP{0x20, 0x1, 0xd, 0xb8, 0x0, 0x1, 0x0, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1} (expected)
        			        != net.IP(nil) (actual)

        				Diff:
        				--- Expected
        				+++ Actual
        				@@ -1,2 +1,2 @@
        				-(net.IP) (len=16 cap=16) 2001:db8:1:1::1
        				+(net.IP) <nil>

looks like that frr uses os/os/architecture specific values in the
zapi message.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@gmail.com>

a16/gobgp,j-kato/gobgp,fujita/gobgp,osrg/gobgp,fujita/gobgp,tamihiro/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,osrg/gobgp,j-kato/gobgp,fujita/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,tamihiro/gobgp,tamihiro/gobgp,osrg/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp