test/sqwait: test retrieving an SQE post io_uring_sqring_wait()

There was a kernel bug in which after calling io_uring_enter(2) with
IORING_ENTER_SQ_WAIT, the application would still be unable to get
a new SQE. The kernel commit has the details:

https://git.kernel.dk/cgit/linux/commit/?h=io_uring-6.12&id=28aabffae6be54284869a91cd8bccd3720041129

Add a test case to test for this.

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