Add tests for figure utility functions

[Schiano-NOAA]

The current helper functions in the utils_plot.R file need testing framework to ensure continue functioning:

• plot_timeseries()
• plot_error
• plot_aa()
• average_age_line()
• cohort_line()
• reference_line()
• axis_breaks()
• cap_first_letter()
• filter_data()
• get_id()
• calculate_reference_point()
• label_magnitude()
• check_grouping()

Some functions like plot_timeseries() and filter_data() may need more than 1 test due to their complexity. Please use the example data in the package (stockplotr::example_data) when data is needed or create your own example data set within the test.