tests: Run all tests through a randomized readdir()

tests: Run all tests through a randomized readdir()

Having undefined (but in practice rarely changing) ordering for
`readdir()` ended up screwing us over for bootloader config
generation; see https://bugzilla.redhat.com/show_bug.cgi?id=1226520

Let's make things significantly more likely to fail more quickly in
the future if similar bugs are introduced.  We accomplish this by
introducing a little `LD_PRELOAD` library that randomizes the results
of `readdir()`.