Rework log fragments adding
Rework log fragments adding

Created LogFragmentsMixin class that which is used to group test
fragments handling logic. It's content is used by LavaResult class to
handle log fragments adding to the groups structure. The main reason of
mixin creation was to separate log fragments handling from test results
parsing.

Signed-off-by: Michal Galka <35351e7ebb7262d28d7902253ba30ca44ccd1bb4@collabora.com>

kernelci/kernelci-backend,kernelci/kernelci-backend