Make Canvas test case independent of chosen font.

0/0 is not always a red pixel after rendering "Hello" in red. The
chosen font has an influence on that. Looking for some red pixel in row
0 is pretty safe.

Change-Id: I669548c3c9d8cd25d720998fd9ba31c1f480e4e3
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>