results.py: Allow partially completed runs to be summarized

This restores the ability to create a summary from a partial set of
runs by using the same functionality as resume.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
backend.py: delete destination for file backends.

If a file backend exists (currently we only have file backends) when a
run starts, delete the original result before starting. This ensures
that running summary on an incomplete run gets the new result and not an
old one.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>