Fix behaviour of TreeSubSet Min and Max

Provide correct override in TreeSubSet for Min and Max, they now take
account of the range of the subset.

As the root is guaranteed to be in the range, then the code does not
need to check something is in the range, if _root is not null.

src/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs

@@ -124,6 +124,54 @@ internal override bool IsWithinRange(T item)
                 return comp >= 0;
             }
 
+            internal override T MinInternal
+            {
+                get
+                {
+                    Node current = _root;
+                    T result = default(T);
+
+                    while (current != null) {
+
+                        int comp = _lBoundActive ? Comparer.Compare(_min, current.Item) : -1;
+                        if (comp == 1) {
+                            current = current.Right;
+                        } else {
+                            result = current.Item;
+                            if (comp == 0)
+                                break;
+                            current = current.Left;
+                        }
+                    }
+
+                    return result;
+                }
+            }
+
+            internal override T MaxInternal
+            {
+                get
+                {
+                    Node current = _root;
+                    T result = default(T);
+
+                    while (current != null) {
+
+                        int comp = _uBoundActive ? Comparer.Compare(_max, current.Item) : 1;
+                        if (comp == -1) {
+                            current = current.Left;
+                        } else {
+                            result = current.Item;
+                            if (comp == 0)
+                                break;
+                            current = current.Right;
+                        }
+                    }
+
+                    return result;
+               }
+            }
+
             internal override bool InOrderTreeWalk(TreeWalkPredicate<T> action)
             {
                 VersionCheck();

src/System.Collections/src/System/Collections/Generic/SortedSet.cs

@@ -1575,7 +1575,9 @@ public int RemoveWhere(Predicate<T> match)
 
         #region ISorted members
 
-        public T Min
+        public T Min => MinInternal;
+
+        internal virtual T MinInternal
         {
             get
             {
@@ -1594,7 +1596,9 @@ public T Min
             }
         }
 
-        public T Max
+        public T Max => MaxInternal;
+
+        internal virtual T MaxInternal 
         {
             get
             {

src/System.Collections/tests/Generic/SortedSet/SortedSet.Generic.cs

@@ -29,7 +29,6 @@ protected override int CreateT(int seed)
 
         protected override bool DefaultValueAllowed => true;
 
-        [ActiveIssue(16760)]
         [Fact]
         public void SortedSet_Generic_GetViewBetween_MinMax()
         {