gsettings test: stop testing long-deprecated API

We want to try taking this away now...

https://bugzilla.gnome.org/show_bug.cgi?id=732102
gsettings: remove long-deprecated "schema" property

This property has been deprecated for three years after only having
existed for one.  We've wanted to reuse the name for all that time, so
let's try to actually remove it now and see if we can get away with it.

https://bugzilla.gnome.org/show_bug.cgi?id=732102
gvariant tests: workaround libc/compiler "issue"

memcmp() is declared by glibc as follows:

  /* Compare N bytes of S1 and S2.  */
  extern int memcmp (const void *__s1, const void *__s2, size_t __n)
       __THROW __attribute_pure__ __nonnull ((1, 2));

despite the fact that it is valid to call it with a null pointer if the
size is zero.

gcc 4.9.0 contains a new optimisation that sees that we pass a pointer
to this function and concludes that it certainly must not be null,
removing a later check and thereby causing a crash.

We protect the invocation of memcmp() with a condition to prevent gcc
from making this false assumption (arguably under wrong advice from
glibc).