selftests: net: bump default cmd() timeout to 20 seconds

selftests: net: bump default cmd() timeout to 20 seconds

We always used 5 sec as the default command timeout. But soon after
it was introduced, David effectively made us ignore the timeout
(it was passed to process.communicate() as the wrong argument).
Gal recently fixed that, but turns out the 5 sec is not enough
for a lot of tests and setups. The fix caused regressions.

In particular running reconfig commands (e.g. XDP attach) on mlx5
with 32 rings and 9k MTU, on a heavily-debug-enabled kernel takes
more than 5 sec. The XDP installation command will time out after
5 sec but since the sleeps in the kernel are non interruptible
the command finishes anyway, leaving the XDP program attached,
but with non-zero exit code. defer()ed cleanups are not installed,
breaking the environment for subsequent tests.

Since "install XDP" is a pretty normal command a "point fix"
does not seem appropriate. 32 rings is a fairly reasonable
config, too, so we should just increase the timeout to 20 sec.

There's no real reason behind the value of 20.

Fixes: 1cf270424218 ("net: selftest: add test for netdev netlink queue-get API")
Fixes: f0bd19316663 ("selftests: net: fix timeout passed as positional argument to communicate()")
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Acked-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Link: https://patch.msgid.link/20260629233348.2145841-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>