glib/tests/refcount.c: Fix tests on non-GCC-isque compilers

Hi,

This attempts to fix the refcount-macro test on non-GCC-isque compilers. From the commit message:

From grefcount.h, in order to disable warnings when G_DISABLE_CHECKS is defined, we need to make use of the GCC-isque-only __extension__ for it to work. This means that in the saturation tests, we can only expect no warnings to show if we are building on GCC-isque compilers with G_DISABLE_CHECKS defined.

This will fix the refcount-macro test on Visual Studio builds at least.

This will be applicable for master and glib-2-60.

With blessings, thank you!