Add testAssert() for arbitrary tests; fix skinLineComments() off-by-1

Add testAssert() for arbitrary tests; fix skinLineComments() passing off-by-1 character to onComment(slice(.., end))

Example of the off-by-1 error:
```
function TestComments() {
      // Bear class
      function Bear(x,y,z) {
        this.position = [x||0,y||0,z||0]
      }

      Bear.prototype.roar = function(message) {
        return 'RAWWW: ' + message; // Whatever
      };

      function Cat() {
      /* 1
         2
         3*/
      }

      Cat.prototype.roar = function(message) {
        return 'MEOOWW: ' + /*stuff*/ message;
      };
    }
  Assertion failed:
 " Bear clas" !== " Bear class"
502 tests run in 59ms
1 failures.
```