Add SubSequence test coverage

Author: Tim Vermeulen

Tests/SwiftAlgorithmsTests/TestUtilities.swift

@@ -350,5 +350,17 @@ func validateIndexTraversals<C>(
         }
       }
     }
+    
+    // slices
+    for (startOffset, start) in indicesIncludingEnd.enumerated() {
+      for (endOffset, end) in indicesIncludingEnd.enumerated().dropFirst(startOffset) {
+        let slice = c[start..<end]
+        
+        XCTAssertEqual(slice.isEmpty, startOffset == endOffset)
+        XCTAssertEqual(slice.count, endOffset - startOffset)
+        XCTAssertEqual(slice.startIndex, start)
+        XCTAssertEqual(slice.endIndex, end)
+      }
+    }
   }
 }