tests: Fix a compile warning

tests: Fix a compile warning

This silences a follow warning.

[  155s] /usr/include/bits/string_fortified.h:106:34: warning: 'char* __builtin_strncpy(char*,
  const char*, unsigned int)' output truncated before terminating nul copying as many bytes
  from a string as its length [-Wstringop-truncation]
[  155s]   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
[  155s]       | ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[  155s] ../tests/tc_screenshooter.cpp: In function 'void client_registry_cb_global(void*,
  wl_registry*, uint32_t, const char*, uint32_t)':

Change-Id: Ibe959f4fdb0117b4b554558d29d79b50da0a6ed9