Leftover from a previous iteration of this code - having a static but
unnecessarily large size for dirname results in:

../test/litest.c:1251:38: warning: ‘snprintf’ output may be truncated before the last format character [-Wformat-truncation=]
   snprintf(dest, sizeof(dest), "%s/%s", dirname, filename);
                                      ^
../test/litest.c:1251:3: note: ‘snprintf’ output 2 or more bytes (assuming 4097) into a destination of size 4096
   snprintf(dest, sizeof(dest), "%s/%s", dirname, filename);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>