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>

gio/tests/converter-stream.c

@@ -1037,12 +1037,11 @@ test_converter_pollable (void)
       if (!is_readable)
 	g_assert_cmpint (res, ==, -1);
 
-      /* After closing the write end, we can't get WOULD_BLOCK any more */
-      if (!socket_out)
-        {
-          g_assert_no_error (error);
-          g_assert_cmpint (res, !=, -1);
-        }
+      /* Even after closing the write end, we can get WOULD_BLOCK (particularly
+       * on FreeBSD), so we can’t make any assertions based on `!socket_out`.
+       * This is because the FIN packets may still be in the out buffer of one
+       * half of the socket pair, while the in buffer of the other half has some
+       * data, but not enough for a full block for the converter to consume. */
 
       if (res == -1)
 	{