Show stdout on failure

[nlsandler]

A handful of improvement to the tests script's output on failure:

1. If return code and stdout are both incorrect, print both of them instead of just return code (fixes Print test program output on failure #20)
2. Use difflib to print difference between expected and actual stdout
3. For optimization tests, print entire assembly function only if it's below a certain length threshold (default 20 instructions). Printing the whole function is useful if it's quite short (as it is for most Chapter 19 tests) but unwieldy if it's very long (as it is for most chapter 20 tests)
4. Cleaned up some bad whitespace in optimization test output