selftests: tc_actions: don't dump 2MB of \0 to stdout

Since we started running selftests in NIPA we have been seeing
tc_actions.sh generate a soft lockup warning on ~20% of the runs.
On the pre-netdev foundation setup it was actually a missed irq
splat from the console. Now it's either that or a lockup.

I initially suspected a socket locking issue since the test
is exercising local loopback with act_mirred.
After hours of staring at this I noticed in strace that ncat
when -o $file is specified _both_ saves the output to the file
and still prints it to stdout. Because the file being sent
is constructed with:

dd conv=sparse status=none if=/dev/zero bs=1M count=2 of=$mirred
^^^^^^^^^

the data printed is all \0. Most terminals don't display nul
characters (and neither does vng output capture save them).
But QEMU's serial console still has to poke them thru which
is very slow and causes the lockup (if the file is >600kB).

Replace the '-o $file' with '> $file'. This speeds the test up
from 2m20s to 18s on debug kernels, and prevents the warnings.

Fixes: ca22da2fbd69 ("act_mirred: use the backlog for nested calls to mirred ingress")
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260214035159.2119699-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>