Here's a simple implementation without sweet.js:

  var inlineTest = function(fn, tests) {
    (tests || []).forEach(function(test){
      if (fn.apply(this, test[0]) !== test[1]) {
        throw new Error('Failed inline unit test with args: ' + test[0]);
      }
    });
    return fn;
  };

  var square = inlineTest(function(n){
    return n * n;
  }, [
    [[2],4], // [[arrayOfArguments], expectedResult]
    [[3],5]  // <-- This test will throw an error
  ]);

EDIT: Change fn.call to fn.apply