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.
That explains a few details I saw in the mirror this morning. I have fixed up the commits, and will hope that is sufficient to undo the damage :pray: