Autotests: Get rid of GENERATE_DATA flag in updating example files process

[AlexeyGirin]

Background
Now we have GENERATE_DATA flag that allow saveToFile function to update file examples if it is true.
At the same time for screenshot update we use --update-snapshots run parameter.
Both of these processes (example file update and screenshot update) has the same nature - the update previously saved example file/screenshots
So it is obvious to unite them and make one universal update procedure

Solution
Screenshot update procedure set process.env.UPDATE_SNAPSHOTS environment variable to '1' if launched
so it obvious to use it for file update procedure as well
Update saveToFile function and remove GENERATE_DATA flag from configs