Fix tests with invalid fds

[Darksonn]

A recent change in the stdlib made it so that two tests no longer pass:

tokio-uring/tests/fs_file.rs

Lines 164 to 176 in 5fb1a4f

	#[test]
	#[ignore]
	fn drop_off_runtime() {
	let file = tokio_uring::start(async {
	let tempfile = tempfile();
	File::open(tempfile.path()).await.unwrap()
	});
	let fd = file.as_raw_fd();
	drop(file);
	assert_invalid_fd(fd);
	}

tokio-uring/tests/fs_file.rs

Lines 132 to 146 in 5fb1a4f

	#[test]
	#[ignore]
	fn explicit_close() {
	let mut tempfile = tempfile();
	tempfile.write_all(HELLO).unwrap();
	tokio_uring::start(async {
	let file = File::open(tempfile.path()).await.unwrap();
	let fd = file.as_raw_fd();
	file.close().await.unwrap();
	assert_invalid_fd(fd);
	})
	}

Instead, the process is killed with an error about the fd being invalid (which is correct - that's what we're testing). The tests need to be fixed, and if there is any unsoundness anywhere that needs to be fixed too.