Fix resultsdb reporting for ARM tests (decompressed images)
    
    For the ARM tests, HDD_1 winds up being somefile.raw, but the
    original image filename is somefile.raw.xz . We need to pass
    the *original* filename to resultsdb_conventions when reporting
    the result, so fedfind will be able to find the image. I could
    have put some more relaxed image finding logic into fedfind for
    this, but I think it's safer just to fix this code to pass the
    correct original filename. It's not too hard to handle this
    case, if we run into others in future, we can reassess.
    
    I also assigned the `TEST_TARGET` value to a variable instead
    of constantly accessing it through the dict, as it makes the
    code a bit more readable, I think.