Add code fix for S3353: Unchanged local variables should be const

[Corniel]

As @martin-strecker-sonarsource was working on #5971, I thought It would be nice to add a fix for this rule (for C#). From the fix's perspective the test case file is a bit messy, so I added two files just to test the fix, one with the pre, and one with the post condition. I did not create an issue (yet) for this code fix.