Add tests

[buster]

Right now, the travis build only builds the library, but it should have some tests to run through.

[jeremyletang]

The problem is that, travis cannot use gtk version ulterior to GTK 3.6, so it's hard to test everything.

[buster]

I think tests can also be built according to cfg attributes. So, one might be able to add tests/gtk.rs and inside split up tests for 3.6, 3.10 etc.

[jeremyletang]

Yes you're right.
Moreover having test will help us to follow which features of gtk are really working. At the moment we have a lot of widgets wrapped inside rgtk, but no test to certify that they are working.