Unit tests

[rhersen]

I wrote some unit tests for commander.c. I had to make exit() a member function of struct command in order to mock it, and I'm also using setjmp/longjmp to stop execution and continue the test suite whenever the mocked exit() is called.

[beakr]

+1

[tj]

+1 as well, just need to review and update this a bit

[stephenmathieson]

idk how i feel about command_t->exit, but needs a rebase either way