Mark test-cs.exe executable so it can be executed directly on Linux. #17.

This is a bit of a hack but it should work on most Linux distributions
where Mono executables are a registered non-native executable file
type. Doing it this way saves us from having to somehow figure out
whether we need to invoke mono.

Unfortunately the test.cs build target won't work on any platform
except Windows and Linux unless we do something more clever.