nixos/test-driver: Make tesseract OCR optional.

By default this is now enabled, and it has to be explicitely enabled
using "enableOCR = true". If it is set to false, any usage of
getScreenText or waitForText will fail with an error suggesting to pass
enableOCR.

This should get rid of the rather large dependency on tesseract which
we don't need for most tests.

Note, that I'm using system("type -P") here to check whether tesseract
is in PATH. I know it's a bashism but we already have other bashisms
within the test scripts and we also run it with bash, so IMHO it's not a
problem here.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>