[varLib.avar] Add roundtrip test

Fails currently.

Tests/varLib/avar_test.py

@@ -1,11 +1,9 @@
+from fontTools.varLib.models import VariationModel
 from fontTools.varLib.avar import _pruneLocations
 import unittest
 import pytest
 
-
+TESTS = [
-@pytest.mark.parametrize(
-    "locations, poles, expected",
-    [
     (
         [
             {"wght": 1},
@@ -44,8 +42,10 @@ import pytest
             {"wght": 0.5, "wdth": 0.5},
         ],
     ),
-    ],
+]
-)
+
+
+@pytest.mark.parametrize("locations, poles, expected", TESTS)
 def test_pruneLocations(locations, poles, expected):
     axisTags = set()
     for location in locations:
@@ -54,6 +54,29 @@ def test_pruneLocations(locations, poles, expected):
 
     locations = [{}] + locations
 
-    output = _pruneLocations(locations, poles, axisTags)
+    pruned = _pruneLocations(locations, poles, axisTags)
 
-    assert output == expected, (output, expected)
+    assert pruned == expected, (pruned, expected)
+
+
+@pytest.mark.parametrize("locations, poles, expected", TESTS)
+def test_roundtrip(locations, poles, expected):
+    axisTags = set()
+    for location in locations:
+        axisTags.update(location.keys())
+    axisTags = sorted(axisTags)
+
+    locations = [{}] + locations
+    expected = [{}] + expected
+
+    model1 = VariationModel(locations, axisTags)
+    model2 = VariationModel(expected, axisTags)
+
+    for location in poles:
+        i = model1.locations.index(location)
+        support1 = model1.supports[i]
+
+        i = model2.locations.index(location)
+        support2 = model2.supports[i]
+
+        assert support1 == support2, (support1, support2)