Ensure we save QA screenshots to an absolute path

What does this MR do?

This allows to open the file address printed in the terminal, otherwise the address started with tmp which was relatove to the qa/ dir.

Why was this MR needed?

Because that was confusing to get a file://tmp/screenshot_2018-02-01-09-22-47.148.png address that couldn't be opened! Now you'll get file:///Users/remy/Code/GitLab/gdk/gitlab/qa/tmp/screenshot_2018-02-01-09-22-47.148.png.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together