[corlib] Fix Array.Sort throwing when fewer keys than items are provided

[corlib] Fix Array.Sort throwing when fewer keys than items are provided

The `Sort<TKey, TValue> (TKey [] keys, TValue [] items, IComparer<TKey> comparer)` overload in Array throwed when keys.Length != items.Length.
However, on .NET you can pass in a keys array with fewer elements than in items and it'd sort the elements in items up to that point.

Fixed our implementation by only throwing when keys.Length > items.Length.

This was revealed by the coreclr/tests/src/CoreMangLib/cti/system/array/arraysort12.exe test.
I decided to port the test into our testsuite so we catch regressions earlier.