Refactor the code

[apjanke]

The test function's code has some issues:

• The main function is several hundred lines long.
• Logic for parsing tests, running tests, and tracking results is interleaved across the whole function.
• All variables are prefixed with "__", hurting readability.
• It uses local functions, which can't be called directly, so they're hard to test.
• Results tracking is handled by seven or eight separate local variables.
• It uses eval logic with generated temporary function strings, which is a little hard to follow.

Same goes for runtests, to a lesser extent.

Refactor the code to address this.

TODO

• [x] Factor out test-running implementation to objects
• [x] Test results summary in runtests2
• [ ] Give runtests2 log file handling (for the lower-level test output)
• [x] More sophisticated input parsing for runtests2, for multiple targets, logging options, etc.
• [x] Have __run_test_suite__ use runtest2's summary output

[apjanke]

I'm deferring logging to a separate issue https://github.com/apjanke/octave-testify/issues/56 because it's a more complicated topic and doesn't block the refactoring itself.

[apjanke]

I'm happy with the refactoring now. Moved test2_refactor and runtests2_refactor back to test2 and runtests2 in https://github.com/apjanke/octave-testify/commit/91d8fe76e71a11cbf1027283400f9b33a15b23ec.