glib/tests/convert.c: Skip tests that aren't meaningful for Windows

Hi,

This is an MR that attempts to fix glib/tests/convert.c running on Windows, as it is actually making some assumptions that are not true for Windows for g_get_filename_charsets(). From the code documentation, setting the G_FILENAME_ENCODING envvar has no effect on Windows because everything is assumed as UTF-8 in g_get_filename_charsets(), so we could just disregard testing g_get_filename_charsets() when setting the G_FILENAME_ENCODING envvar.

With blessings, thank you!