glib-networking: use correct error code in ptest

glib-networking: use correct error code in ptest

The eagain patch is currently using G_IO_ERROR_BUSY as part of the check
to retry when the simul_read_thread test fails during ptests, but the
actual error code is 27, which corresponds to G_IO_ERROR_WOULD_BLOCK.
Change the check so that it looks for the right code.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>