Add IOExtensions tests

SabreTools.IO/IOExtensions.cs

@@ -1,5 +1,4 @@
-using System;
+using System.Collections.Generic;
-using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Text;

SabreTools.Test/IO/IOExtensionsTests.cs

@@ -0,0 +1,24 @@
+using SabreTools.IO;
+using Xunit;
+
+namespace SabreTools.Test.IO
+{
+    public class IOExtensionsTests
+    {
+        [Theory]
+        [InlineData(null, null)]
+        [InlineData("", null)]
+        [InlineData("     ", null)]
+        [InlineData("no-extension", null)]
+        [InlineData("NO-EXTENSION", null)]
+        [InlineData("no-extension.", null)]
+        [InlineData("NO-EXTENSION.", null)]
+        [InlineData("filename.ext", "ext")]
+        [InlineData("FILENAME.EXT", "ext")]
+        public void NormalizedExtensionTest(string path, string expected)
+        {
+            string actual = path.GetNormalizedExtension();
+            Assert.Equal(expected, actual);
+        }
+    }
+}