Work around -Wformat-truncation=/-Wformat-overflow= warnings (#356)

Work around -Wformat-truncation=/-Wformat-overflow= warnings (#356)

sprintf(3)/snprintf(3) destination buffers need to be large enough
so that gcc doesn't warn -Wformat-truncation= or -Wformat-overflow=
when source buffer size is 1024 bytes.

--
../test/test_syscalls.c:1445:47: warning: '%s' directive output may be truncated writing 1 byte into a region of size between 0 and 1023 [-Wformat-truncation=]
 #define PATH(p)  (snprintf(path, sizeof path, "%s/%s", testdir, p), path)
                                               ^~~~~~~
../test/test_syscalls.c:1458:19:
  res = mkdir(PATH("a"), 0755);
                   ~~~

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>