equalWithPrecision assertion always passes

[IvanSanchez]

When trying to whip up some unit tests for #4726, I noticed that the behaviour of the equalWithPrecision assertion defined in test/node_modules/mapbox-gl-js-test.js is pretty much broken.

I mean, I can go to https://github.com/mapbox/mapbox-gl-js/blob/master/test/unit/style-spec/function.test.js#L77 , and change

t.equalWithPrecision(f(2), 30 / 9, 1e-6);

into

t.equalWithPrecision(f(2), 123456789, 1e-6);

and the unit tests will still pass.