tests/fetch: Add test for broken mirror tarball
    
    With PREMIRRORS set and BB_NO_NETWORK = "1" bitbake should not try to
    fetch into non-initialized git directory if tarball is broken (or not in
    gzip format)
    
    [Yocto 14822]
    
    Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
    Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
    Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>