Add missing tests for fitness & equilibrium functions

[dfalster]

Despite extensive tests, somehow various functions related to fitness and demographic equilibrium went untested. Presumably because they were slow. Lack of tests here enabled loss of functionality as we changed some of the interface.

This PR adds some tests on master, which will then be ported back to develop and updated as needed.

Here is the list of functions idnetified

Equilbirum.R

• [x] equilibrium_birth_rate
• [x] equilibrium_birth_rate_iteration
• [x] equilibrium_birth_rate_solve
• [x] equilibrium_birth_rate_solve_robust
• [x] equilibrium_birth_rate_hybrid
• [ ] make_equilibrium_runner
• [ ] equilibrium_runner_cleanup
• [ ] equilibrium_birth_rate_solve_target

fitness_support

• [x] bounds
• [x] check_bounds
• [x] bounds_infinite
• [x] check_point
• [x] positive_1d
• [x] positive_1d_bracket (tested via running positive_1d)

fitness_trait_space

• [x] max_fitness
• [x] viable_fitness

Fitness

• [x] fitness_landscape
• [x] max_growth_rate
• [x] carrying_capacity [function removed]