Repo.groovy

// Class under test
class Repo {
    
    static listRepos() {
        def dir = new File("repos")
        def repos = []
        dir.listFiles().each {
            def regex = /repos\/(.+)\.git/
            def match = it =~ regex
            if(match.size() > 0) {
                repos.push(match[0][1])
            }
        }
        return repos
    }

}

someTests.groovy

// Snippet of test

    void testShowRefBranch() {
        
        def mockRepo = new MockFor(Repo, true)
        
        mockRepo.demand.refName(1..1) { type, ref ->
            Repo.refName(type, ref)
        }

        mockRepo.use() {
            assertEquals(model, controller.showRef())
        }
    }