tests/signals: Disable large enumeration value test that is failing on PPC64

Basically due to a combination of va_args semantics around
signed/unsigned ints, this test case fails on ppc64.  At the moment,
we have as yet to find any real-world consumer with such a large
enumeration value.

Unfortunately, the possible fixes for this are extremely invasive;
we would have to define a new enum API.

Given both of these facts, we believe it makes the most sense at the
current time to simply not test this. If we at a later time determine
there is such a real-world consumer, we can look at doing the
necessary fixes.

https://bugzilla.gnome.org/show_bug.cgi?id=686662