tests/fetch: Add tests for premirror using real project
    
    Existing test uses fake local repo for better performance. Adding
    test which uses real (but still small) project hosted on
    yoctoproject.org.
    
    Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>