* Leave a byte that starts no UTF-8 sequence at the end of a console write
`find_last_utf8_boundary` read every byte at or above 0xc0 as the start of a
sequence, giving 0xf8..=0xff an expected length of 4 and cutting the tail back
to before it. `_find_last_utf8_boundary` stops the four-byte range at 0xf8 and
returns the full length for anything above it, leaving the byte for
MultiByteToWideChar to answer with U+FFFD.
`write_console_utf8` therefore reported fewer bytes written than it was given
whenever one of those bytes fell in the last three, and for a buffer that began
with one it cut the length to 0, converted nothing and returned Ok(0) -- a
write that never advances.
The scan also ran a fourth iteration the C loop does not; the arms happen to
answer the same there, but the bound is now the same 3.
* Drop the expectedFailure on test_winconsoleio's test_write
The marker went stale with the boundary fix in the parent commit. The test
writes `b'\xff'*10` and `data + b'\xff'` to CONOUT$ and asserts the returned
count, which is the case `find_last_utf8_boundary` was answering wrong, so
regrtest now reports it as an unexpected success.
Jeong, YunWon