QUIC: fixed ngx_http_test_reading() for QUIC streams.

QUIC: fixed ngx_http_test_reading() for QUIC streams.

Previously this function generated an error trying to figure out if client shut
down the write end of the connection.  The reason for this error was that a
QUIC stream has no socket descriptor.  However checking for eof is not the
right thing to do for an HTTP/3 QUIC stream since HTTP/3 clients are expected
to shut down the write end of the stream after sending the request.

Now the function handles QUIC streams separately.  It checks if c->read->error
is set.  The error flags for c->read and c->write are now set for all streams
when closing the QUIC connection instead of setting the pending_eof flag.