0031958: Tests - testdiff shows empty Total Image difference

0031958: Tests - testdiff shows empty Total Image difference

stat(img1) counter is now incremented in case of total log.

Renamed "Total Image difference" -> "Total IMAGE difference" for consistency with other output.

Regexp inserting links to images now looks for "IMAGE ..." at the start of line
to avoid misdetection on "Total IMAGE difference" line.
_diff_show_positive_ratio() now prints image difference percents
without leading empty space to highlight large deviations in RED.

Image files having no match are now added to statistics as differences.