buildbot: add a checkbox to add an additional step that runs the test suite

[mojca (Mojca Miklavec)]

We would generally not enable running the test suite on the buildbot. Many ports don't even support that, many require a lot of resources to run the suite. But it would be nice to add a checkbox next to the portlist field, so that developers could manually trigger an additional step.

We could optionally add some rule later to determine for which ports the test suite could/should be running. But at the moment it would be nice to be able to run them manually on older boxes.