transformer: don't shell out for filter commands and make the tests work on Windows

Instead of passing the filter commands to the shell, split them with the shlex module. This gives us more control and a more similar behaviour on unix/win32 systems. I could only find two users in Debian which just call python with some script, so this seems safe to me.

Adjust the tests accordingly and use the running python executable to test the functionality as we can be sure that's available on all platforms. Also add a new test for the symbol filter command which wasn't tested before.