Improve character heal_rate tests

Describe plainly how healing works, in terms of the Character functions
that calculate healing rate per turn, and enforce the current behavior
with simple test cases.

The `healing_rate` and `healing_rate_medicine` functions have the
following behavior, as tested:

Baseline healing is 8.64 HP per day while sleeping, 0 while awake. This
base rate comes from `data/json/game_balance.json : PLAYER_HEALING_RATE`.

Traits like "Poor healer", "Regeneration", and "Rapid Metabolism" affect
healing rate, with different multipliers for awake and asleep rates.

Hidden health stat `[-200, +200]` influences healing rate, with +200
doubling healing, and -200 giving no healing at all, even while asleep.

Bandages and/or disinfectant each add 1-3 HP/day to healing rate while
awake, doubled to 2-6 HP/day while sleeping. The head gets less benefit
while the torso gets the most benefit from these treatments; the limbs
all get the same moderate benefit.

When bandages and disinfectant are used together, not only do their
effects combine, they are again doubled, giving +4-12 HP/day while
awake (head 4, limbs 8, torso 12), and +8-24 HP/day while asleep.

Changes in this commit:

- Add top-level documentation and coverage notes
- Use constants for hp_per_day, tolerance, and effects
- Include descriptions for all trait/mutation healing factors
- Add heal_rate_medicine tests for bandages and/or disinfectant