Add LevelCalc unit test

Geekbot.net/Lib/LevelCalc.cs

@@ -4,7 +4,7 @@ using System.Linq;
 
 namespace Geekbot.net.Lib
 {
-    internal class LevelCalc : ILevelCalc
+    public class LevelCalc : ILevelCalc
     {
         private int[] _levels;
 

Tests/Lib/LevelCalc.test.cs

@@ -0,0 +1,49 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Geekbot.net.Lib;
+using Xunit;
+
+namespace Tests.Lib
+{
+    public class LevelCalc_test
+    {
+        public static IEnumerable<object[]> LevelCalcTestData
+        {
+            get
+            {
+                yield return new object[]
+                {
+                    500,
+                    13
+                };
+                
+                yield return new object[]
+                {
+                    41659,
+                    55
+                };
+                
+                yield return new object[]
+                {
+                    0,
+                    1
+                };
+                
+                yield return new object[]
+                {
+                    4000000,
+                    101
+                };
+            }
+        }
+
+
+        [Theory, MemberData(nameof(LevelCalcTestData))]
+        public async Task GetLevel(int messages, int expectedResult)
+        {
+            var levelCalc = new LevelCalc();
+            var result = levelCalc.GetLevel(messages);
+            Assert.Equal(result, expectedResult);
+        }
+    }
+}