* [feaLib.ast] fix checking multiple markClass definitions don't redefine same glyphs
As pointed out by @mhosken, we are looking in the wrong list for glyphs
that have already been defined in a previous markClass definition.
With this patch, the current markClass.fea test case fails becuase it defines 'acute'
twice for the same @TOP_MARKS class:
fontTools.feaLib.error.FeatureLibError: Lib/fontTools/feaLib/testdata/markClass.fea:6:5: Glyph acute already defined at Lib/fontTools/feaLib/testdata/markClass.fea:3:1
Also see conversation at:
3b79d51755 (r94622074)
* feaLib/testdat/markClass.fea: remove duplicate 'acute' in @TOP_MARKS
Now the test pass, after 45c77b7
* [feaLib.builder_test] test case for redefined glyph in multiple markClass definitions