glib/tests/: Avoid GCCisms and fix tests for MSVC

glib/tests/: Avoid GCCisms and fix tests for MSVC

Skip the tests on inf/nan strings for the gvariant and strfuncs tests, and
skip the hex strings for the strtod tests in strfuncs as they are C99
features that are not yet supported by Visual C++ (even 2013).  Use a
definition for NAN and INFINITY (that is also used in PyGObject) as
atof("NaN") and atof("Infinity") simply returns 0.0 (which is not a NAN)
in Visual C++ to fix the tests running there.

Also adapt to the format of g_ascii_formatd() when dealing with 1e99.

https://bugzilla.gnome.org/show_bug.cgi?id=711047