Record statistics on which test covers which condition

[rillig]

Assuming that gobco would output which test covers which condition, it would be possible to answer questions like the following:

• For each testee, is there a test that covers all branches of that testee? (minimal unit tests)
• Which tests are necessary to get 100% branch coverage for a given testee?

Implementation ideas:

• Have an additional list of test names for each condition, possibly indirectly by mapping the test name to an int for performance.
• Extend the gobco JSON output with this list of tests.