tests: net: tcp: IPv4 header was not initialized

When creating IPv4 packet, the IPv4 header contained uninitialized
values. This was reported by valgrind.

Conditional jump or move depends on uninitialised value(s)
   at 0x8066AB0: calc_chksum (utils.c:386)
   by 0x8066C99: calc_chksum_pkt (utils.c:428)
   by 0x806710A: net_calc_chksum (utils.c:487)
   by 0x8067B8D: net_calc_chksum_icmpv4 (net_private.h:138)
   by 0x8067FEC: net_icmpv4_set_chksum (icmpv4.c:90)
   by 0x8068E7A: net_icmpv4_send_error (icmpv4.c:338)
   by 0x807272F: send_icmp_error (connection.c:773)
   by 0x80731A3: net_conn_input (connection.c:992)
   by 0x806A34D: net_ipv4_process_pkt (ipv4.c:212)
   by 0x805D8B9: process_data (net_core.c:103)
   by 0x805D939: processing_data (net_core.c:117)
   by 0x805E498: net_rx (net_core.c:300)
 Uninitialised value was created by a stack allocation
   at 0x804A437: setup_ipv4_tcp (main.c:297)

Conditional jump or move depends on uninitialised value(s)
   at 0x8067117: net_calc_chksum (utils.c:489)
   by 0x8067B8D: net_calc_chksum_icmpv4 (net_private.h:138)
   by 0x8067FEC: net_icmpv4_set_chksum (icmpv4.c:90)
   by 0x8068E7A: net_icmpv4_send_error (icmpv4.c:338)
   by 0x807272F: send_icmp_error (connection.c:773)
   by 0x80731A3: net_conn_input (connection.c:992)
   by 0x806A34D: net_ipv4_process_pkt (ipv4.c:212)
   by 0x805D8B9: process_data (net_core.c:103)
   by 0x805D939: processing_data (net_core.c:117)
   by 0x805E498: net_rx (net_core.c:300)
   by 0x805E51D: process_rx_packet (net_core.c:312)
   by 0x8085E87: work_q_main (work_q.c:39)
 Uninitialised value was created by a stack allocation
   at 0x804A437: setup_ipv4_tcp (main.c:297)

Fixes #7447

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>