tests: net: ipv6_fragment: Improve test_recv_ipv6_fragment test case
tests: net: ipv6_fragment: Improve test_recv_ipv6_fragment test case

Fix some problems with test_recv_ipv6_fragment test case:
* net_pkt_ipv6_hdr_prev was not set, in result reassembly logic
  overwritten wrong part of the IPv6 header
* Wrong checksum was set in the first fragment

Add more verification to the test_recv_ipv6_fragment test case:
* Actually register ICMPv6 Echo Reply handler so that the reassembled
  packet can be analyzed
* Verify that IPv6, ICMPv6 headers and data are correct

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr