Here is a within ggplot
method, using ..count..
and ..PANEL..
:
ggplot(test, aes(test2)) +
geom_bar(aes(y = (..count..)/tapply(..count..,..PANEL..,sum)[..PANEL..])) +
facet_grid(~test1)
As this is computed on the fly, it should be robust to changes to plot parameters.