tests: net: ipv6_fragment: Fix llvm compiler warning
tests: net: ipv6_fragment: Fix llvm compiler warning

llvm complains about char* -> u8_t* type conversion.

tests/net/ipv6_fragment/src/main.c:645:52: warning:
  passing 'char [11]' to parameter of type 'const u8_t *' (aka
  'const unsigned char *') converts between pointers to integer
  types with different sign [-Wpointer-sign]
        bool written = net_pkt_append_all(pkt, data_len, data,

Jira: ZEP-2274

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

GiulianoFranchetto/zephyr,explora26/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,fractalclone/zephyr-riscv,galak/zephyr,Vudentz/zephyr,zephyriot/zephyr,galak/zephyr,punitvara/zephyr,sharronliu/zephyr,Vudentz/zephyr,explora26/zephyr,runchip/zephyr-cc3220,holtmann/zephyr,rsalveti/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyriot/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,runchip/zephyr-cc3220,fractalclone/zephyr-riscv,fbsder/zephyr,ldts/zephyr,mbolivar/zephyr,runchip/zephyr-cc3220,explora26/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,kraj/zephyr,finikorg/zephyr,zephyriot/zephyr,fbsder/zephyr,holtmann/zephyr,rsalveti/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,kraj/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,fractalclone/zephyr-riscv,nashif/zephyr,zephyriot/zephyr,fractalclone/zephyr-riscv,galak/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,holtmann/zephyr,fbsder/zephyr,mbolivar/zephyr,sharronliu/zephyr,nashif/zephyr,nashif/zephyr,aceofall/zephyr-iotos,nashif/zephyr,rsalveti/zephyr,punitvara/zephyr,holtmann/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,rsalveti/zephyr,zephyriot/zephyr,fbsder/zephyr,rsalveti/zephyr,holtmann/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,sharronliu/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,fbsder/zephyr,explora26/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,runchip/zephyr-cc3220,punitvara/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,fractalclone/zephyr-riscv,mbolivar/zephyr,ldts/zephyr,sharronliu/zephyr,aceofall/zephyr-iotos,kraj/zephyr,Vudentz/zephyr,mbolivar/zephyr,kraj/zephyr