The concept of what was unexpected was typically defined as "some
depencendy wasn't installed", which isn't unexpected at all as it's
totally optional. Since it confuses new contributors as they think
something is wrong with their installation it seems sensible to get
rid of the concept.

This change also adds the concept of optional tests that are required
to work on a specific platform(s) (e.g. test_winreg on Windows). This
should help catch compile issues instead of a test being blindly
skipped even when it should have run.

The skipped test list in the future can also print out the reason for
being skipped to make it more obvious as to why the skipping occurred.