selftests/tc-testing: Enable CONFIG_IP_SET

The config snippet specifies CONFIG_NET_EMATCH_IPSET. This option
depends on CONFIG_IP_SET.

Set CONFIG_IP_SET to be enabled at part for tc-testing.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250630153341.Wgh3SzGi@linutronix.de


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