tests: improved screenshot test to get tempdir using tempfile module.
tests: improved screenshot test to get tempdir using tempfile module.

myself659/splinter,underdogio/splinter,nikolas/splinter,underdogio/splinter,cobrateam/splinter,objarni/splinter,nikolas/splinter,lrowe/splinter,drptbl/splinter,underdogio/splinter,gjvis/splinter,gjvis/splinter,lrowe/splinter,gjvis/splinter,bmcculley/splinter,bmcculley/splinter,bmcculley/splinter,nikolas/splinter,objarni/splinter,myself659/splinter,drptbl/splinter,cobrateam/splinter,myself659/splinter,cobrateam/splinter,drptbl/splinter,lrowe/splinter,objarni/splinter