tests: net: lwm2m_engine: workaround stack overflow on qemu_x86

When running on qemu_x86, the test test_socket_state failed on
stack overflow. Setting CONFIG_TEST_EXTRA_STACK_SIZE to 1024
seems to fix the issue. So add a board specific config for
qemu_x86 so this test would pass.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>