Test examples on CI

[chapulina]

The examples are not tested on CI, so it's easy to forget to update them. We should be testing them on CI like other libraries do:

• ign-gazebo
• ign-gui
• ign-math

I think at least some of the libraries above require the library to be installed before running these tests, which is not ideal. I believe there are ways to work around this though, by setting environment variables, or installing, but to a local directory.

[ahcorde]

Already merged