web: add StepTree tests
web: add StepTree tests

I was previously unaware of this test suite, but it seems like a good
place to improve coverage.

Signed-off-by: Aidan Oldershaw <30ef7a6e0d016bc846f26d89dacbd23aceded206@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse