It's a weird issue. I have disabled tests on non-native ABIs for now but I think it's worth looking into.
If you build glib-networking for the first time, everything works fine. Once 32-bit version is installed, the libproxy tests error out with:
ERROR: libproxy - missing test plan
ERROR: libproxy - exited with status 133 (terminated by signal 5?)
Tried forcing in-source build and moving glib-pacrunner. Only removing /usr/lib32/gio/* makes tests work again.
*** Bug 529836 has been marked as a duplicate of this bug. ***