selftests: netfilter: nft_flowtable.sh: bump socat timeout to 1m

selftests: netfilter: nft_flowtable.sh: bump socat timeout to 1m

Now that this test runs in netdev CI it looks like 10s isn't enough
for debug kernels:
  selftests: net/netfilter: nft_flowtable.sh
  2024/05/10 20:33:08 socat[12204] E write(7, 0x563feb16a000, 8192): Broken pipe
  FAIL: file mismatch for ns1 -> ns2
  -rw------- 1 root root 37345280 May 10 20:32 /tmp/tmp.Am0yEHhNqI
 ...

Looks like socat gets zapped too quickly, so increase timeout to 1m.

Could also reduce tx file size for KSFT_MACHINE_SLOW, but its preferrable
to have same test for both debug and nondebug.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240511064814.561525-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>