tests: Remove an incorrect assertion in converter-stream

While the assertion always turned out to be true on Linux, it frequently caused spurious test failures on FreeBSD.

After some remote debugging, I think the cause is as written up in the comment in the code in this commit. However, I cannot be certain, as the more debugging messages I added, the harder the failure was to reproduce; and I don’t have access to a FreeBSD machine.

This fixes failures like:

Bail out! GLib-GIO:ERROR:../gio/tests/converter-stream.c:1043:test_converter_pollable: assertion failed (error == NULL): Resource temporarily unavailable (g-io-error-quark, 27)

It’s succeeded 1000 times in a row on the FreeBSD CI now; previously it was failing one time in three: https://gitlab.gnome.org/GNOME/glib/-/jobs/1936395.

Signed-off-by: Philip Withnall pwithnall@endlessos.org