[libcxx] [test] Fix an improper assumption about Null Forward Iterators.

[STL_MSFT]

Summary

[libcxx] [test] Fix an improper assumption about Null Forward Iterators.

Value-initialized iterators still can't be compared to those with parents.

[STL_MSFT]

I could mark these assertions as libcxx-specific, but I tend to believe that they should be removed outright (even if the natural implementation of a list without debug checks provides this behavior).

[EricWF]

LGTM. Go ahead and remove the tests. I filed a libc++ bug reminding me to diagnose those comparisons in debug mode (https://llvm.org/bugs/show_bug.cgi?id=31343).

test/std/containers/sequences/list/iterators.pass.cpp

#if TEST_STD_VER >= 11
         C::iterator i = c.begin();
         i->first = 3;
         C::const_iterator j = i;
         assert(j->first == 3);
     }
 #endif
 #if TEST_STD_VER > 11
     {
-        std::list<int> c;
         std::list<int>::iterator ii1{}, ii2{};
         std::list<int>::iterator ii4 = ii1;
         std::list<int>::const_iterator cii{};
         assert ( ii1 == ii2 );
         assert ( ii1 == ii4 );
 
         assert (!(ii1 != ii2 ));
 
         assert ( (ii1 == cii ));
         assert ( (cii == ii1 ));
         assert (!(ii1 != cii ));
         assert (!(cii != ii1 ));
-
-        assert ( ii1 != c.cbegin());
-        assert ( cii != c.begin());
     }
 #endif
 
 }