selftests: tc-testing: fix nat regex matching

selftests: tc-testing: fix nat regex matching

In iproute 6.14, the nat ip mask logic was fixed to remove an undefined
behaviour[1]. So now instead of reporting '0.0.0.0/32' on x86 and potentially
'0.0.0.0/0' in other platforms, it reports '0.0.0.0/0' in all platforms.

[1] https://lore.kernel.org/netdev/20250306112520.188728-1-torben.nielsen@prevas.dk/

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://patch.msgid.link/20250401144908.568140-1-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>