selftests: Add FIB onlink tests

selftests: Add FIB onlink tests

BugLink: http://bugs.launchpad.net/bugs/1786352
[ Upstream commit 153e1b84f477f716bc3f81e6cfae1a3d941fc7ec ]

Add test cases verifying FIB onlink commands work as expected in
various conditions - IPv4, IPv6, main table, and VRF.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>