selftests: net: add missing config for big tcp tests

The big_tcp test-case requires a few kernel knobs currently
not specified in the net selftests config, causing the
following failure:

  # selftests: net: big_tcp.sh
  # Error: Failed to load TC action module.
  # We have an error talking to the kernel
...
  # Testing for BIG TCP:
  # CLI GSO | GW GRO | GW GSO | SER GRO
  # ./big_tcp.sh: line 107: test: !=: unary operator expected
...
  # on        on       on       on      : [FAIL_on_link1]

Add the missing configs

Fixes: 6bb382bc ("selftests: add a selftest for big tcp")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/all/21630ecea872fea13f071342ac64ef52a991a9b5.1706282943.git.pabeni@redhat.com/


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