Test: Add a test with a single file with lots of extended attributes

[Stebalien]

Now that we try reading extended attributes into a 4KiB buffer before we even enter the allocation loop, we need to improve the tests to actually exercise the allocation loop. Unfortunately, we may run into filesystem limitations...

Alternatively, we could build/test with a "small buffer" feature that shrinks this 4KiB buffer to, say, 4 bytes so we always bypass it and hit the allocation loop path.