Non-zero exit code on missing

[TheZoq2]

Right now, it seems like only failing tests get a non-zero exit code. However, when running in pre-commit hooks or CI, I would like missing tests to also give a non-zero exit code since it likely means I either forgot to add an expect file, or forgot to check if the missing tests are fine

[rachitnigam]

The right way to handle this is probably making the exit code depend on the value of the -o filter. If -o fail is provided, then the exit code is zero if we only have missing files. Otherwise, it is the number of missing tests and the number of failed tests.

This can be done by changing the execute_and_summarize method to return a tuple with the failing tests and missing tests and, selecting the right one based on the value of -o filter. @TheZoq2 let me know if you want to take a shot at this and need help