Add more unit tests for coverage

[fredjaya]

Tests required for the following.

pooling_strategies.R

• pool_max_size()
• pool_target_number()

pool_strat_methods.R

• print.pool_strat()

catch_distributions.R

• nb_catch()
• pois_catch()

util.R

• mu_sigma_linknorm() - this stop() is only likely encountered if using the function directly. Keep, but add a no cov
• ev()
• Repeated warning for increasing max_s #23
• optimise_random_prevalence()
• optim_local_int()
• cost_fi_random()
• cost_fi_cluster_random()

design_effect.R

• design_effect_random()

fisher_information.R

• Some cases in fi_pool_cluster() - prioritise last, aside from:
• fi_pool_cluster() when not real_scale
• fi_pool_random()
• fi_pool_cluster_random()

[fredjaya]

coverage back up to ~90%, remaining uncovered lines are mainly warnings()