Fix data race in transformer tests
Fix data race in transformer tests

Don't use shared BeforeEach for StepsRunner tests

[#156776776](https://www.pivotaltracker.com/story/show/156776776)

Signed-off-by: Edwin Xie <6bc24e9c160281972837d9e13287696d26612564@pivotal.io>

cloudfoundry-incubator/executor