Update & fix testing example

[ttimbers]

The test suite example is more complex than it needs to be (like the test data is bigger than needed) and there are some bugs in the test code. This was all fixed in the version we presented at the SSC workshop: https://github.com/UBC-STAT/ssc-workshop-2024/blob/main/slides/03-testing-your-pkg-code.qmd

We also in that version of the testing example made nice, small demos of the test input and output data as part of the planning, for example this slide:

We should present the examples one-by-one like this in the book (as welll as summarize them in the table afterwards).