add tolerance in layout slider push margins test
add tolerance in layout slider push margins test

... to make it pass on etpinard's laptop running Ubuntu 18.04
    and Chrome 69.

aburato/plotly.js,plotly/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,etpinard/plotly.js,aburato/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,plotly/plotly.js