Updated TestReplaceExtensioni, TestFilename and TestFileAndExt

Updated TestReplaceExtensioni, TestFilename and TestFileAndExt

Updated the test cases in TestReplaceExtension to reflect the intent
of ReplaceExtension which is to accept a path and return only the file
name with a new extension. It's intentionally designed to strip out the
path and only provide the filename

Additional test cases have been added to both TestFilename and
TestFileAndExt to clarify behaviour with relative paths.