io_uring: fix regression

[ianic]

Fixes #19451 which describes io_uring regression introduced in #18757.

First commit adds failing test from the issue which fails with panic: @memcpy arguments have non-equal lengths.

1/1 IoUring.test.failing test for issue 19451... thread 248254 panic: @memcpy arguments have non-equal lengths
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/std/os/linux/IoUring.zig:289:45: 0x103d50a in copy_cqes_ready (test)
        @memcpy(cqes[0..count], self.cq.cqes[head..tail]);
                                            ^
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/std/os/linux/IoUring.zig:276:36: 0x103d1c4 in copy_cqes (test)
        return self.copy_cqes_ready(cqes);
                                   ^
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/compiler/IoUring.zig:4251:40: 0x103df00 in test.failing test for issue 19451 (test)
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/compiler/test_runner.zig:158:25: 0x107847c in mainTerminal (test)
        if (test_fn.func()) |_| {
                        ^
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/compiler/test_runner.zig:35:28: 0x104088b in main (test)
        return mainTerminal();
                           ^
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/std/start.zig:501:22: 0x103e579 in posixCallMainAndExit (test)
            root.main();
                     ^
/usr/local/zig/zig-linux-x86_64-0.12.0-dev.3496+a2df84d0f/lib/std/start.zig:253:5: 0x103e0e1 in _start (test)
    asm volatile (switch (native_arch) {
    ^
???:?:?: 0x0 in ??? (???)
error: the following test command crashed:

Second commit is one character fix.

Third simplifies logic in copy_sqes and adds test which touches both paths in copys_sqes.

[andrewrk]

Thanks, nice work on coming up with a unit test.