Consider storing state of which extra credit tests are enabled

[johnvtan]

I think that if I implement an extra credit feature, I'll want those tests enabled every other time I run the test script, especially if later chapters add more tests for old extra credit features (e.g., Ch 7 implements new tests for the goto extra credit from Ch 6).

It would be nice if instead of passing each individual extra credit flag to the script every time I run it, I could run something like

test_compiler --enable-extra-credit goto

That would cause the script to enable the goto tests for all future runs of the script. And then a corresponding --disable-extra-credit flag that would reverse the change.

[nlsandler]

I like this idea! I think it would be easy enough to store a list of enabled features in an .extra-credit file in the root of the repo.