Additional stack-machine.js tests

Added all of my unit tests to the Node.js testfile.

The Node.js implementation fails on 3 of the 6 tests:
```
* stackMachine('0')   == -1 != 0
* stackMachine('01*') == -1 != 0
* stackMachine('88*1+97**1+') == 4096 != -1 // along with the other overflow tests
```