test-network: re-enable test_macsec

test-network: re-enable test_macsec

The outstanding kernel panic should be already fixed in recent enough
kernels by [0]. To make the test safe to run anywhere, let's implement
a simple kernel version check and run the test only if we're running
with at least kernel 6.x. The patch might be in some 5.x kernels as
well, but let's be on the safe side and use 6.x as a baseline here
(which is currently the case for Arch and Fedora Rawhide anyway).

[0] https://lore.kernel.org/netdev/7b3fd03e1a46047e5ffe2a389fe74501f0a93206.1656519221.git.sd@queasysnail.net/T/#u