Fix failing FF layout tests

Some of the new layout tests fail in Firefox: http://127.0.0.1:8124/webodf/tests/tests.html?suite=LayoutTests

* Modified chosen font pt size to avoid FF vs Chrome font size rounding differences
* Allow multiple accepted values in LayoutTests to cope with the fact that FF converts the vertical align property into px, whilst Chrome leaves it as a percentage.
The old code also dealt with `a` being just a number (unit in pixels), which would fail here. No idea why support for pure number was added/needed (i.e. which computed style or node property would be just given as number). 
Tried to come up with some code proposal to also support that, but noone found without blowing up the code too much, and that without a given need, so dropped it. Still worth a note in the review :)