test: Skip sandbox/stat_filename where "stat64" syscall defined

On 32-bit architectures where Linux provides the "stat64" system call,
including i386, the sandbox is unable to filter calls to stat() as glibc uses
this system call itself internally and the sandbox must allow it without
restriction.

Update the sandbox unit tests to skip the "sandbox/stat_filename" test on
systems where the "stat64" system call is defined and the test is certain to
fail.  Also reorder the "#if" statement's clauses to correspond with the
comment preceding it, for clarity.

src/test/test_sandbox.c

@@ -331,13 +331,13 @@ struct testcase_t sandbox_tests[] = {
   SANDBOX_TEST_IN_SANDBOX(rename_filename),
 
 /* Currently the sandbox is unable to filter stat() calls on systems where
- * glibc implements this function using the legacy "stat" system call, or where
- * glibc version 2.33 or later is in use and the newer "newfstatat" syscall is
- * available.
+ * glibc implements this function using either of the legacy "stat" or "stat64"
+ * system calls, or where glibc version 2.33 or later is in use and the newer
+ * "newfstatat" syscall is available.
  *
  * Skip testing sandbox_cfg_allow_stat_filename() if it seems the likely the
  * function will have no effect and the test will therefore not succeed. */
-#if !defined(__NR_newfstatat) && (!defined(__NR_stat) || defined(__NR_stat64))
+#if !defined(__NR_stat) && !defined(__NR_stat64) && !defined(__NR_newfstatat)
   SANDBOX_TEST_IN_SANDBOX(stat_filename),
 #else
   SANDBOX_TEST_SKIPPED(stat_filename),