test_filejournal.cc: use strncpy and terminate with '\0'

CID 966632 (#1 of 1): Copy into fixed size buffer (STRING_OVERFLOW)
 2. fixed_size_dest: You might overrun the 200 byte fixed-size string
 "path" by copying "args[0UL]" without checking the length.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>