Attribute Macro runtime_executable

#[runtime_executable]

Run test case when the executable existing

 // write as example in examples/*rs
 test_with::runner!(exe);
 #[test_with::module]
 mod exe {
     // `/bin/sh` executable exists
     #[test_with::runtime_executable(/bin/sh)]
     fn test_executable_with_path() {
         assert!(true);
     }
 }