Add separate unit test for piecewise-constant check

The previous commit causes the 'curve' parsing validation to preempt
createExpression()'s check for piecewise-constant zoom function using
'step' interpolation, because `line-dasharray`'s style spec type,
`array<number>`, already isn't interpolatable.

Adding this unit test to make sure we always enforce step curves, even
for a theoretical style property whose base type is interpolatable.

debug/index.html

@@ -26,6 +26,10 @@
     hash: true
 });
 
+map.on('load', function () {
+    map.setPaintProperty('road-path', 'line-dasharray', ["curve", ["linear"], ["zoom"], 0, ["literal", [1, 2]], 1, ["literal", [3, 4]]])
+})
+
 </script>
 </body>
 </html>

test/unit/style-spec/expression.test.js

@@ -0,0 +1,21 @@
+'use strict';
+
+const test = require('mapbox-gl-js-test').test;
+const {createExpression} = require('../../../src/style-spec/expression');
+
+test('createExpression', (t) => {
+    test('require piecewise-constant zoom curves to use "step" interpolation', (t) => {
+        const expression = createExpression([
+            'curve', ['linear'], ['zoom'], 0, 0, 10, 10
+        ], {
+            type: 'number',
+            function: 'piecewise-constant'
+        });
+        t.equal(expression.result, 'error');
+        t.equal(expression.errors.length, 1);
+        t.equal(expression.errors[0].message, 'interpolation type must be "step" for this property');
+        t.end();
+    });
+
+    t.end();
+});