g_icon_new_for_string() docs states that it should return a single name
when created with a single name. I add a second condition to this case:
the themed icon must not include default fallbacks (i.e. it must not
have been created with `g_themed_icon_new_with_default_fallbacks()`).
Otherwise the return value of `g_icon_new_for_string()` would not
recreate the same icon list when passed to `g_icon_new_for_string()`
(which would be another documentation inconsistency).

g_icon_new_for_string() is now back to old behavior for this specific
case.

I also revert the unit test for this case, and add a new unit test when
using g_themed_icon_new_with_default_fallbacks() with a single name as
well.

Closes #1513.