Test commands do not need quote escapes on Windows

Another one for Windows compatibility. In essence, on Windows we do not need to escape quotes in the test run command.

My approach was to `let` bind the correct function depending on OS and leave the mapping in. For Windows the `identity` function is mapped.

The Travis CI results show that this does not work on Emacs version 24.3. Any ideas why it is just failing for that version?

Also, I was trying to add a test for this, but found no `test-quote-escape-in-test-run` or similar in the test suite. Also, I do not quite know how I would go about to test this function. Should we refactor the quote escape mapping to a separate function that just returns the test command string (so we can test it separately)?