fix: Make MockDirectory.Exists behavior with forward slash on Windows consistent with actual file system (#1245)

# Changes
- Removed Unix OS check in `MockDirectory.Exists`
- Added additional tests for `MockDirectory`, `MockDirectoryInfo`, `FileSystem.Directory`

Author: tbui17

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockDirectory.cs

@@ -174,7 +174,7 @@ public override void Delete(string path, bool recursive)
     /// <inheritdoc />
     public override bool Exists(string path)
     {
-        if (path == "/" && XFS.IsUnixPlatform())
+        if (path == "/")
         {
             return true;
         }

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockDirectoryTests.cs

@@ -2181,7 +2181,7 @@ public async Task Move_Directory_DoesNotThrow_When_Target_Directory_Parent_Exist
         string sourceDirName,
         string targetDirName)
     {
-        // Arange
+        // Arrange
         var fileSystem = new MockFileSystem();
         fileSystem.Directory.CreateDirectory(sourceDirName);
 
@@ -2192,4 +2192,16 @@ public async Task Move_Directory_DoesNotThrow_When_Target_Directory_Parent_Exist
         await That(fileSystem.Directory.Exists(targetDirName)).IsTrue();
         await That(fileSystem.Directory.Exists(sourceDirName)).IsFalse();
     }
+
+    [Test]
+    public async Task MockDirectory_Exists_ShouldReturnTrue_IfArgIsFrontSlashAndRootDirExists()
+    {
+        string testDir = XFS.Path(@"c:\foo\bar\");
+        var fileSystem = new MockFileSystem(new Dictionary<string, MockFileData>
+        {
+            { testDir,  new MockDirectoryData() }
+        });
+
+        await That(fileSystem.Directory.Exists("/")).IsEqualTo(true);
+    }
 }