Reduce the number of repo creation for tests
    
    For writting further tests, developer has to consider if the fake repo
    for test case should be created only once, or each test requires fake
    repos.
    
    The creation could be controlled by two class variables.
    
    * require_test_repos: indicate if test case requires fake repos.
    * create_repo_per_test: indicate if fake repos should be created for
      each test or only the test case.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>