subset_test: add test for dropping 'rvrn' feature and all FeatureVariationRecords

Added a TODO for the partial dropping of FeatureVariationRecords
https://github.com/fonttools/fonttools/issues/1881#issuecomment-619415044

Tests/subset/subset_test.py

@@ -733,7 +733,8 @@ class SubsetTest(unittest.TestCase):
         self.assertEqual(ttf.flavor, None)
 
 
-def test_subset_feature_variations():
+@pytest.fixture
+def featureVarsTestFont():
     fb = FontBuilder(unitsPerEm=100)
     fb.setupGlyphOrder([".notdef", "f", "f_f", "dollar", "dollar.rvrn"])
     fb.setupCharacterMap({ord("f"): "f", ord("$"): "dollar"})
@@ -753,7 +754,11 @@ def test_subset_feature_variations():
     fb.save(buf)
     buf.seek(0)
 
-    font = TTFont(buf)
+    return TTFont(buf)
+
+
+def test_subset_feature_variations_keep_all(featureVarsTestFont):
+    font = featureVarsTestFont
 
     options = subset.Options()
     subsetter = subset.Subsetter(options)
@@ -771,6 +776,30 @@ def test_subset_feature_variations():
     assert "dollar.rvrn" in font.getGlyphOrder()
 
 
+def test_subset_feature_variations_drop_all(featureVarsTestFont):
+    font = featureVarsTestFont
+
+    options = subset.Options()
+    options.layout_features.remove("rvrn")  # drop 'rvrn'
+    subsetter = subset.Subsetter(options)
+    subsetter.populate(unicodes=[ord("f"), ord("$")])
+    subsetter.subset(font)
+
+    featureTags = {
+        r.FeatureTag for r in font["GSUB"].table.FeatureList.FeatureRecord
+    }
+    glyphs = set(font.getGlyphOrder())
+
+    assert "rvrn" not in featureTags
+    assert glyphs == {".notdef", "f", "dollar"}
+    # all FeatureVariationRecords were dropped
+    assert not font["GSUB"].table.FeatureVariations.FeatureVariationRecord
+
+
+# TODO test_subset_feature_variations_drop_from_end_empty_records
+# https://github.com/fonttools/fonttools/issues/1881#issuecomment-619415044
+
+
 def test_subset_single_pos_format():
     fb = FontBuilder(unitsPerEm=1000)
     fb.setupGlyphOrder([".notdef", "a", "b", "c"])