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 <jukka.rissanen@linux.intel.com>