tests: virtual-stb: Add chromium `--disable-gpu` flag
tests: virtual-stb: Add chromium `--disable-gpu` flag

This fixes the virtual-stb tests since the latest Chromium release
(59.0.3071.109). Without this, Chromium would render an empty white
page when run under `stbt virtual-stb`.

I've also added `--temp-profile` for further isolation. Without it, if I
run `chromium-browser` on my laptop it opens a new tab inside the
virtual-stb instead of starting a separate instance of Chromium.

LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester