test: add regression tests for fixed file table bugs

Add two new test cases:

file-alloc-range-hint: verifies that auto-allocated fixed file slots
stay within the configured IORING_REGISTER_FILE_ALLOC_RANGE, even
after explicit install/remove operations on slots outside the range
corrupt alloc_hint.

pipe-direct-fixed: verifies that IORING_OP_PIPE with specific fixed
file slots (not IORING_FILE_INDEX_ALLOC) returns the correct 0-based
slot indices, communication works through those slots, and unrelated
slots are not accidentally clobbered.

Signed-off-by: Jens Axboe <axboe@kernel.dk>