Add a test for 2 trailing commas in the module export list

testsuite/tests/parser/should_fail/ExportCommaComma.hs

+
+module ExportCommaComma (id, reverse,,) where
+

testsuite/tests/parser/should_fail/ExportCommaComma.stderr

+
+ExportCommaComma.hs:2:38: parse error on input `,'

testsuite/tests/parser/should_fail/all.T

@@ -78,3 +78,4 @@ test('ParserNoMultiWayIf', when(compiler_lt('ghc', '7.5'), skip), compile_fail,
 test('T5425', normal, compile_fail, [''])
 test('T984', normal, compile_fail, [''])
 test('T7848', normal, compile_fail, ['-dppr-user-length=100'])
+test('ExportCommaComma', normal, compile_fail, [''])