Directory.Move behaves differently on Unix vs. Windows (dotnet#37121)

* Directory.Move behaves differently on Unix vs. Windows

We will now throw an exception in Unix platforms like in Windows, if the destination path already exists.

* Modifying ExistingDirectory test method that verifies Move overwrite so its called by all platforms.

We already had a test method that tests Move when the destination path exists. I simply removed the platform attribute that restricted it to Windows and renamed the test method so its universal (as opposed to platform specific).

src/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs

@@ -349,6 +349,13 @@ public static void MoveDirectory(string sourceFullPath, string destFullPath)
                 destFullPath = PathInternal.TrimEndingDirectorySeparator(destFullPath);
             }
 
+            if (FileExists(destFullPath))
+            {
+                // Some Unix distros will overwrite the destination file if it already exists.
+                // Throwing IOException to match Windows behavior.
+                throw new IOException(SR.Format(SR.IO_AlreadyExists_Name, destFullPath));
+            }
+
             if (Interop.Sys.Rename(sourceFullPath, destFullPath) < 0)
             {
                 Interop.ErrorInfo errorInfo = Interop.Sys.GetLastErrorInfo();

src/System.IO.FileSystem/tests/Directory/Move.cs

@@ -315,8 +315,8 @@ public void UnixWhitespacePath()
         }
 
         [Fact]
-        [PlatformSpecific(TestPlatforms.Windows)]  // Moving to existing directory causes IOException
-        public void WindowsExistingDirectory()
+        // Moving to existing directory causes IOException
+        public void ExistingDirectory()
         {
             DirectoryInfo testDir = Directory.CreateDirectory(GetTestFilePath());
             string testDirSource = Path.Combine(testDir.FullName, GetTestFileName());