Fix legacy function conversion tests

[jfirebaugh]

The tests introduced in mapbox/mapbox-gl-js#7095 are failing in native.

Since native converts legacy functions to expressions internally, we want to get all these passing. The first step is to implement support in the test runner, similarly to mapbox/mapbox-gl-js@466162f#diff-a4463a66781c68614988a85564196f32.

[jfirebaugh]

I started porting mapbox/mapbox-gl-js#7095, but beyond #12864 it gets really tedious and error prone to mimic the exact output of gl-js in this area. We already have a working conversion system in gl-native -- just not one that serializes to expression forms that are compatible with these tests. I think it might be better to pull these tests out into a separate suite that doesn't require identical expression serialization.

[stale]

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