asynctest: a stream write refused at submit marks the handle failed (true-async/php-async#315)

[EdmondDantes]

Eight cmocka cases for true-async/php-async#315. A fire-and-forget write refused at submit must set ZEND_ASYNC_IO_WRITE_FAILED on the handle before its free_cb runs, hand the buffer back once and leave an exception:

• a write over the 0x7ffff000-byte cap, and a write on a closed handle;
• a vectored write with unsupported flags, and one on a closed handle;
• a write and a vectored write that uv_write refuses at once (a read-only pipe);
• a vectored write of 65536 slots, which releases every slot;
• the control: a write that succeeds leaves the flag clear.

The comment on ZEND_ASYNC_IO_WRITE_FAILED in Zend/zend_async_API.h now says it covers refusals and stream handles only; no ABI change.

7 of 8 fail against php-async main (c95dcf7), 8 of 8 pass against php#315, 3 runs; under valgrind the test loses nothing and leaves no descriptor open. Built with --enable-asynctest; no CI job runs this SAPI.