selftests: Add test cases for FDB flush with VXLAN device

Test all the supported arguments for FDB flush. The test checks
configuration, not traffic. Note that the flag 'offloaded' is not checked
as it is not relevant when there is no hardware.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>