Move some test code from the examples into coveralls.

[mhammond]

Some of the examples have code which is really better described as "test" code, and so tends to detract from the example itself - I'm thinking "Callbacks" in particular, and possibly rondpoint (but that's already more a fixture than an example anyway). #431 added a new "coverall" fixture which probably is a better home for such code.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-55