Updated DiamondTests.cs

`Rows` method adjusted to use system dependent new line characters.

Author: spirit11

exercises/practice/diamond/DiamondTests.cs

@@ -7,7 +7,7 @@
 public class DiamondTests
 {
     public static readonly char[] AllLetters = GetLetterRange('A', 'Z');
-    private static string[] Rows(string x) => x.Split(new[] { '\n' }, StringSplitOptions.None);
+    private static string[] Rows(string x) => x.Split(Environment.NewLine);
 
     private static string LeadingSpaces(string x) => x.Substring(0, x.IndexOfAny(AllLetters));
     private static string TrailingSpaces(string x) => x.Substring(x.LastIndexOfAny(AllLetters) + 1);
@@ -125,4 +125,4 @@ public static Arbitrary<char> Chars()
     {
         return Arb.Default.Char().Filter(x => x >= 'A' && x <= 'Z');
     }
-}
+}