tests: Run all tests through a randomized readdir()

tests: Run all tests through a randomized readdir()

Having undefined ordering (but in practice rarely changing)
ordering for `readdir()` ended up screwing us over with respect
to bootloader config file read ordering.

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()`.