selftests: vxlan_mdb: Avoid duplicate test names

Rename some test cases to avoid overlapping test names which is
problematic for the kernel test robot. No changes in the test's logic.

Suggested-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240227170418.491442-1-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>