Improved test for roll_the_die

 - Tests now check whether multiple rolls supply different results
 - RollDie is checked for returning all values in the expected range
 - Increased for loop from 100 times to 1000 times to prevent false negative. Encountered that 100 rolls was not sufficient to have all numbers in range(1, 18) to be generated.

Rationale:

My first implementation (thanks XKCD) made the test suite pass.

```
public int RollDie()
{
  return 4; // chosen by fair dice roll.
            // guaranteed to be random.
}
```

Author: Han Bol

exercises/concept/roll-the-die/RollTheDieTests.cs

@@ -1,25 +1,38 @@
 using Xunit;
 using Exercism.Tests;
+using System.Collections.Generic;
 
 public class RollTheDieTests
 {
     [Fact]
     [Task(1)]
     public void RollDie()
     {
+        var rollCount = 1000;
+        var rolls = new HashSet<int>(rollCount);
         var player = new Player();
-        for (var i = 0; i < 100; i++)
+        for (var i = 0; i < rollCount; i++)
         {
+            var roll = player.RollDie();
+            rolls.Add(roll);
             Assert.InRange(player.RollDie(), 1, 18);
         }
+        Assert.Equal(18, rolls.Count);
     }
 
     [Fact]
     [Task(2)]
     public void GenerateSpellStrength()
     {
+        var rollCount = 100;
+        var rolls = new HashSet<double>(rollCount);
         var player = new Player();
-        var strength = player.GenerateSpellStrength();
-        Assert.InRange(strength, 0.0, 100.0);
+        for (var i = 0; i < rollCount; i++)
+        {
+            var strength = player.GenerateSpellStrength();
+            rolls.Add(strength);
+            Assert.InRange(strength, 0.0, 100.0);
+        }
+        Assert.Equal(rollCount, rolls.Count);
     }
 }