[unittests][Support] AIX: Skip sticky bit file tests

On AIX, attempting (without root) to set the sticky bit on a file with
the `chmod` utility will give:
```
chmod: not all requested changes were made to <file>
```

The same occurs when modifying other permission bits on a file with the
sticky bit already set.

It seems that the `chmod` function will report success despite failing
to set the sticky bit.

llvm-svn: 350735