Break up toy model/distro tests into small chunks

[MichaelClerx]

Most toy models / distributions currently have a single method that tests several things. These should be broken up into proper unit tests

• [ ] test_toy_annulus_logpdf.py
• [ ] test_toy_br_ap_model.py
• [ ] test_toy_cone_logpdf.py
• [ ] test_toy_constant_model.py
• [ ] test_toy_fitzhugh_nagumo_model.py
• [ ] test_toy_gaussian_logpdf.py
• [ ] test_toy_goodwin_oscillator_model.py
• [ ] test_toy_hes1_michaelis_menten_model.py
• [ ] test_toy_hh_ik_model.py
• [ ] test_toy_high_dimensional_gaussian_logpdf.py
• [ ] test_toy_logistic_model.py
• [ ] test_toy_lotka_volterra_model.py
• [ ] test_toy_multimodal_gaussian_logpdf.py
• [ ] test_toy_neals_funnel_logpdf.py
• [ ] test_toy_parabolic_error.py
• [ ] test_toy_repressilator.py
• [ ] test_toy_rosenbrock.py
• [ ] test_toy_simple_egg_box_logpdf.py
• [ ] test_toy_sir_model.py
• [ ] test_toy_stochastic_degradation_model.py
• [ ] test_toy_twisted_gaussian_logpdf.py
• [x] simple harmonic oscillator