Add tests to cover BB_FETCH_PREMIRRORONLY functionality
    
    Basic test to cover functionality of BB_FETCH_PREMIRRORONLY using
    local git repository. Local repository has been chosen to allow easy
    manipulation with the repo to simulate behaviour reported in
    [Yocto 13233]
    
    Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>