Add render test for format expression nested under decision expression

[alexshalamov]

Render test for mapbox/mapbox-gl-native#14254
GL native fix: mapbox/mapbox-gl-native#14259

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality

[asheemmamoowala]

Are separate tests needed for nested format expressions with text-scale and text-font as well?

[alexshalamov]

Are separate tests needed for nested format expressions with text-scale and text-font as well?

I don't think so, I quickly made few tests with font-scale and text-font options and these are not exercising code that I'm adding in #8068, Chris made quite a good test suite for layout specific properties. If I find that something is missing, I would definitely add more unit / render tests.